feat(Board): add CandidateCard mini switcher

Reviewed-on: #50

.oxlintrc.json

@@ -0,0 +1,195 @@
+{
+  "$schema": "./node_modules/oxlint/configuration_schema.json",
+  "plugins": ["import"],
+  "categories": {
+    "correctness": "error",
+    "suspicious": "warn",
+    "perf": "warn",
+    // style/restriction off: opt-in, contradictory grab-bags. Wanted rules enabled individually below.
+    "style": "off",
+    "restriction": "off"
+  },
+  "env": {
+    "browser": true,
+    "es2021": true
+  },
+  "ignorePatterns": [
+    "node_modules",
+    "dist",
+    "build",
+    ".svelte-kit",
+    ".vercel",
+    "*.config.js",
+    "*.config.ts"
+  ],
+  "rules": {
+    "no-console": "warn",
+    "no-debugger": "error",
+    "no-alert": "warn",
+
+    // no-cycle resolves $-aliases via tsconfig auto-discovery (no resolver config in oxlint)
+    "import/no-cycle": "error",
+    "import/no-duplicates": "warn",
+    "import/no-unassigned-import": "off", // CSS/side-effect imports are intentional
+
+    "no-sequences": "error",
+    "no-underscore-dangle": "off",
+    "no-shadow": "warn",
+    "no-implicit-coercion": "warn",
+    "no-await-in-loop": "warn",
+    "no-return-assign": "warn",
+    "no-new": "warn",
+    "no-unneeded-ternary": "warn"
+  },
+  // FSD boundaries. oxlint has no zone rule, so layer/segment direction is enforced
+  // with no-restricted-imports patterns scoped per glob. Layer order (high->low):
+  // app(exempt top shell) > routes > widgets > features > entities > shared.
+  // A layer bans imports from itself (cross-slice via alias) and every layer above.
+  // Overrides are LAST-WINS, not merged: a file matching two overrides keeps only the
+  // last rule config. So the domain override (below) is a self-contained superset, and
+  // the test/story override (last) fully disables boundary checks for those files.
+  "overrides": [
+    // shared = lowest layer: imports nothing above it
+    {
+      "files": ["src/shared/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": [
+                "$app",
+                "$app/*",
+                "$routes",
+                "$routes/*",
+                "$widgets",
+                "$widgets/*",
+                "$features",
+                "$features/*",
+                "$entities",
+                "$entities/*"
+              ],
+              "message": "FSD layer violation: `shared` is the lowest layer and may not import from any layer above it."
+            }
+          ]
+        }]
+      }
+    },
+    // entities: import shared only; no other entity via alias; interior ui<-only-ui
+    {
+      "files": ["src/entities/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": ["$app", "$app/*", "$routes", "$routes/*", "$widgets", "$widgets/*", "$features", "$features/*"],
+              "message": "FSD layer violation: `entities` may only import from `shared`."
+            },
+            {
+              "group": ["$entities", "$entities/*"],
+              "message": "FSD cross-slice violation: do not import another entity via its alias. Use relative imports inside your own slice; invert the dependency through a higher layer for cross-slice needs."
+            },
+            {
+              "group": ["../ui", "../ui/*", "../../ui/*"],
+              "message": "FSD segment violation: only `ui` may import `ui`. Interior direction is ui -> model -> domain."
+            }
+          ]
+        }]
+      }
+    },
+    // features: import entities/shared only; no other feature via alias
+    {
+      "files": ["src/features/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": ["$app", "$app/*", "$routes", "$routes/*", "$widgets", "$widgets/*"],
+              "message": "FSD layer violation: `features` may only import from `entities` and `shared`."
+            },
+            {
+              "group": ["$features", "$features/*"],
+              "message": "FSD cross-slice violation: do not import another feature via its alias. Invert the dependency through a higher layer (widget/route)."
+            },
+            {
+              "group": ["../ui", "../ui/*", "../../ui/*"],
+              "message": "FSD segment violation: only `ui` may import `ui`. Interior direction is ui -> model -> domain."
+            }
+          ]
+        }]
+      }
+    },
+    // widgets: import features/entities/shared only; no other widget via alias
+    {
+      "files": ["src/widgets/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": ["$app", "$app/*", "$routes", "$routes/*"],
+              "message": "FSD layer violation: `widgets` may only import from `features`, `entities`, and `shared`."
+            },
+            {
+              "group": ["$widgets", "$widgets/*"],
+              "message": "FSD cross-slice violation: do not import another widget via its alias. Invert the dependency through the route layer."
+            },
+            {
+              "group": ["../ui", "../ui/*", "../../ui/*"],
+              "message": "FSD segment violation: only `ui` may import `ui`. Interior direction is ui -> model -> domain."
+            }
+          ]
+        }]
+      }
+    },
+    // routes: top of the FSD list, imports any layer below; only app is above it
+    {
+      "files": ["src/routes/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            { "group": ["$app", "$app/*"], "message": "FSD layer violation: `routes` may not import from `app`." }
+          ]
+        }]
+      }
+    },
+    // domain (FSD+): pure logic. Imports NO layer (not even shared) and no sibling
+    // model/ui segment. Superset: wins over the layer override above for these files.
+    {
+      "files": ["src/**/domain/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": [
+                "$app",
+                "$app/*",
+                "$routes",
+                "$routes/*",
+                "$widgets",
+                "$widgets/*",
+                "$features",
+                "$features/*",
+                "$entities",
+                "$entities/*",
+                "$shared",
+                "$shared/*"
+              ],
+              "message": "FSD+ domain isolation: `domain` is pure business logic and may not import any layer (including `shared`). Allowed: relative imports within `domain` and framework-agnostic npm packages."
+            },
+            {
+              "group": ["../model", "../model/*", "../../model/*", "../ui", "../ui/*", "../../ui/*"],
+              "message": "FSD+ domain isolation: `domain` may not import sibling `model` or `ui` segments. Dependency flows ui -> model -> domain, never back."
+            }
+          ]
+        }]
+      }
+    },
+    // tests/stories/fixtures legitimately cross-import (e.g. $entities/Font/testing).
+    // Must be LAST so last-wins disables boundary checks for them.
+    {
+      "files": ["**/*.test.ts", "**/*.spec.ts", "**/*.stories.svelte", "src/**/testing/**"],
+      "rules": {
+        "no-restricted-imports": "off"
+      }
+    }
+  ]
+}

e2e/preview-text.test.ts

@@ -1,3 +1,4 @@
+import { windowSizeForLine } from '../src/entities/Font/domain/windowSizeForLine/windowSizeForLine';
 import {
     expect,
     test,
@@ -5,12 +6,22 @@ import {
 
 test.describe('preview text', () => {
     test('drives the slider character rendering', async ({ comparison }) => {
+        /**
+         * Must stay a single unwrapped line of ASCII: the assertion feeds
+         * `text.length` (UTF-16 code units) to `windowSizeForLine`, but the
+         * renderer feeds it the line's grapheme count. They match only for
+         * plain ASCII — emoji/combining marks (length > graphemes) or wrapping
+         * (one input string splitting into several lines) silently desync them.
+         */
+        const text = 'Sphinx';
         await comparison.pickPair('Inter', 'Roboto');
-        await comparison.setPreviewText('Sphinx');
+        await comparison.setPreviewText(text);
 
-        // Each grapheme renders as a `.char-wrap` cell in the slider once
-        // both fonts are loaded. Six glyphs → six cells.
-        await expect(comparison.slider.locator('.char-wrap')).toHaveCount(6);
+        // Window chars render as `.char-wrap` cells for crossfade. The window
+        // size is a pure function of the line's grapheme count — assert against
+        // the rule, not a hardcoded constant, so tuning the policy can't silently
+        // break this. "Sphinx" is one unwrapped line of 6 graphemes.
+        await expect(comparison.slider.locator('.char-wrap')).toHaveCount(windowSizeForLine(text.length));
     });
 
     test('preserves the typed value in the input', async ({ comparison }) => {

oxlint.json

@@ -1,29 +0,0 @@
-{
-  "plugins": ["import"],
-  "categories": {
-    "correctness": "error",
-    "suspicious": "warn",
-    "perf": "warn",
-    "style": "warn",
-    "restriction": "error"
-  },
-  "env": {
-    "browser": true,
-    "es2021": true
-  },
-  "ignore": [
-    "node_modules",
-    "dist",
-    "build",
-    ".svelte-kit",
-    ".vercel",
-    "*.config.js",
-    "*.config.ts"
-  ],
-  "rules": {
-    "no-console": "off",
-    "no-debugger": "error",
-    "no-alert": "warn",
-    "import/no-cycle": "error"
-  }
-}

package.json

@@ -6,8 +6,7 @@
   "type": "module",
   "sideEffects": [
     "*.css",
-    "**/router.ts",
-    "**/bindings.svelte.ts"
+    "**/router.ts"
   ],
   "scripts": {
     "dev": "vite",
@@ -49,7 +48,6 @@
     "@types/jsdom": "28.0.1",
     "@vitest/browser-playwright": "4.1.5",
     "@vitest/coverage-v8": "4.1.5",
-    "bits-ui": "2.18.1",
     "clsx": "^2.1.1",
     "dprint": "0.54.0",
     "jsdom": "29.1.1",

src/app/providers/QueryProvider.svelte

@@ -6,7 +6,7 @@
     descendants of this provider.
 -->
 <script lang="ts">
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
 import { QueryClientProvider } from '@tanstack/svelte-query';
 import type { Snippet } from 'svelte';
 
@@ -18,6 +18,9 @@ interface Props {
 }
 
 let { children }: Props = $props();
+
+// First call to the lazy singleton — constructs the shared client for the app.
+const queryClient = getQueryClient();
 </script>
 
 <QueryClientProvider client={queryClient}>

src/entities/Font/api/proxy/proxyFonts.test.ts

@@ -19,7 +19,9 @@ vi.mock('$shared/api/api', () => ({
 }));
 
 import { api } from '$shared/api/api';
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
+
+const queryClient = getQueryClient();
 import { fontKeys } from '$shared/api/queryKeys';
 import { FontResponseError } from '../../lib/errors/errors';
 import {

src/entities/Font/api/proxy/proxyFonts.ts

@@ -11,7 +11,7 @@
  */
 
 import { api } from '$shared/api/api';
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
 import { fontKeys } from '$shared/api/queryKeys';
 import { buildQueryString } from '$shared/lib/utils';
 import type { QueryParams } from '$shared/lib/utils';
@@ -26,7 +26,7 @@ import type { UnifiedFont } from '../../model/types';
  */
 export function seedFontCache(fonts: UnifiedFont[]): void {
     fonts.forEach(font => {
-        queryClient.setQueryData(fontKeys.detail(font.id), font);
+        getQueryClient().setQueryData(fontKeys.detail(font.id), font);
     });
 }
 

src/entities/Font/domain/index.ts

@@ -8,3 +8,4 @@ export {
     findSplitIndex,
     type LineRenderModel,
 } from './computeLineRenderModel/computeLineRenderModel';
+export { windowSizeForLine } from './windowSizeForLine/windowSizeForLine';

src/entities/Font/domain/windowSizeForLine/windowSizeForLine.test.ts

@@ -0,0 +1,38 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { windowSizeForLine } from './windowSizeForLine';
+
+describe('windowSizeForLine', () => {
+    it('returns 0 for an empty or non-positive line', () => {
+        expect(windowSizeForLine(0)).toBe(0);
+        expect(windowSizeForLine(-3)).toBe(0);
+    });
+
+    it('floors non-empty short lines at the minimum window of 1', () => {
+        expect(windowSizeForLine(1)).toBe(1);
+        expect(windowSizeForLine(2)).toBe(1);
+        expect(windowSizeForLine(3)).toBe(1);
+    });
+
+    it('scales with round(n / 3) in the mid range', () => {
+        expect(windowSizeForLine(6)).toBe(2);
+        expect(windowSizeForLine(12)).toBe(4);
+    });
+
+    it('caps at the maximum window of 5', () => {
+        expect(windowSizeForLine(15)).toBe(5);
+        expect(windowSizeForLine(16)).toBe(5);
+        expect(windowSizeForLine(100)).toBe(5);
+    });
+
+    it('rounds to nearest at fractional boundaries', () => {
+        // round(4/3)=1, round(5/3)=2, round(13/3)=4, round(14/3)=5
+        expect(windowSizeForLine(4)).toBe(1);
+        expect(windowSizeForLine(5)).toBe(2);
+        expect(windowSizeForLine(13)).toBe(4);
+        expect(windowSizeForLine(14)).toBe(5);
+    });
+});

src/entities/Font/domain/windowSizeForLine/windowSizeForLine.ts

@@ -0,0 +1,39 @@
+/**
+ * Crossfade-window sizing policy for the dual-font slider.
+ *
+ * The slider renders a band of per-char `Character` cells that opacity-crossfade
+ * between the two fonts; everything outside the band is committed native bulk
+ * text. A fixed band looked wrong on short lines — a 6-grapheme line left almost
+ * no bulk, so nearly the whole line shimmered as per-char DOM. The band size
+ * therefore scales with the line's grapheme count and caps so long lines don't
+ * pay for an oversized per-char DOM band.
+ */
+
+/**
+ * Fraction of a line's graphemes that sit in the crossfade band.
+ */
+const WINDOW_RATIO = 1 / 3;
+/**
+ * Smallest band for a non-empty line — guarantees at least one crossfading char.
+ *
+ * Accepted tradeoff: short lines now get a band of 1–2, so a fast slider drag
+ * can unmount a char before its ~100ms opacity crossfade finishes, a slight pop.
+ * Worth it for the "bulk committed, small band shimmering" look on short lines;
+ * raising this trades that pop back for less committed bulk.
+ */
+const WINDOW_MIN = 1;
+/**
+ * Largest band regardless of line length — bounds per-char DOM cost.
+ */
+const WINDOW_MAX = 5;
+
+/**
+ * Crossfade window size, in graphemes, for a line of `n` graphemes.
+ * `clamp(round(n / 3), 1, 5)`; an empty/non-positive line gets no window.
+ */
+export function windowSizeForLine(n: number): number {
+    if (n <= 0) {
+        return 0;
+    }
+    return Math.min(WINDOW_MAX, Math.max(WINDOW_MIN, Math.round(n * WINDOW_RATIO)));
+}

src/entities/Font/index.ts

@@ -1,29 +1,92 @@
-export * from './domain';
-export * from './lib';
-export * from './ui';
+export {
+    computeLineRenderModel,
+    DualFontLayout,
+    findSplitIndex,
+    windowSizeForLine,
+} from './domain';
+export type {
+    ComparisonLine,
+    ComparisonResult,
+    LineRenderModel,
+} from './domain';
 
-// Pure model surface (types + constants) is part of the convenient top-level
-// API. Stateful stores are deliberately excluded — see below.
-export * from './model/const/const';
-export * from './model/types';
+export {
+    createFontRowSizeResolver,
+    FontNetworkError,
+    FontResponseError,
+    getFontUrl,
+} from './lib';
+export type { FontRowSizeResolverOptions } from './lib';
+
+export {
+    FontApplicator,
+    FontSampler,
+    FontVirtualList,
+} from './ui';
+
+// Pure model surface (types + constants).
+export {
+    DEFAULT_FONT_SIZE,
+    DEFAULT_FONT_WEIGHT,
+    DEFAULT_LETTER_SPACING,
+    DEFAULT_LINE_HEIGHT,
+    FONT_SIZE_STEP,
+    FONT_WEIGHT_STEP,
+    LETTER_SPACING_STEP,
+    LINE_HEIGHT_STEP,
+    MAX_FONT_SIZE,
+    MAX_FONT_WEIGHT,
+    MAX_LETTER_SPACING,
+    MAX_LINE_HEIGHT,
+    MIN_FONT_SIZE,
+    MIN_FONT_WEIGHT,
+    MIN_LETTER_SPACING,
+    MIN_LINE_HEIGHT,
+    VIRTUAL_INDEX_NOT_LOADED,
+} from './model/const/const';
+export type {
+    FilterGroup,
+    FilterType,
+    FontCategory,
+    FontCollectionFilters,
+    FontCollectionSort,
+    FontCollectionState,
+    FontFeatures,
+    FontFilters,
+    FontLoadRequestConfig,
+    FontLoadStatus,
+    FontMetadata,
+    FontProvider,
+    FontStyleUrls,
+    FontSubset,
+    FontVariant,
+    FontWeight,
+    FontWeightItalic,
+    UnifiedFont,
+    UnifiedFontVariant,
+} from './model/types';
+
+/*
+ * Stores are exposed as lazy accessors / classes (not eager singletons): the
+ * entity's public API is complete, so consumers go through this barrel instead
+ * of deep-importing `./model` (FSD public-API boundary). Construction happens on
+ * first call, so this is inert at import. The slice root already transitively
+ * loads `@tanstack/query-core` via `./ui` (FontVirtualList), so surfacing the
+ * stores here adds no new eager cost.
+ */
+export {
+    FontLifecycleManager,
+    FontsByIdsStore,
+    getFontCatalog,
+    getFontLifecycleManager,
+} from './model';
+export type { FontCatalogStore } from './model';
 
 /*
  * `./api` (proxy clients: `fetchProxyFonts`, `seedFontCache`, …) is intentionally
- * NOT re-exported here. Those clients import `$shared/api/queryClient`, whose
- * module eval runs `new QueryClient()` and loads `@tanstack/query-core`. Funneling
- * them through this barrel made every consumer of `$entities/Font` — including
- * pure-domain and type-only importers — eager-load TanStack and construct the
- * client (notably in unit specs). Import API clients via the segment:
- *   import { fetchProxyFonts } from '$entities/Font/api';
- */
-
-/*
- * Stores (`fontCatalogStore`, `fontLifecycleManager`, `FontsByIdsStore`) are
- * intentionally NOT re-exported here. They instantiate module-level singletons
- * and pull `@tanstack/query-core`, so funneling them through this barrel would
- * make every consumer of `$entities/Font` eager-instantiate stores (and break
- * tree-shaking / test init-order). Import them via the model segment:
- *   import { fontCatalogStore } from '$entities/Font/model';
+ * NOT re-exported here — those are not part of the entity's consumed surface and
+ * importing them eagerly constructs the TanStack `queryClient`. Import via the
+ * segment: `import { fetchProxyFonts } from '$entities/Font/api'`.
  */
 
 // `./testing` is intentionally not re-exported: fixtures must not leak into the

src/entities/Font/model/index.ts

@@ -1,6 +1,51 @@
-export * from './const/const';
+export {
+    DEFAULT_FONT_SIZE,
+    DEFAULT_FONT_WEIGHT,
+    DEFAULT_LETTER_SPACING,
+    DEFAULT_LINE_HEIGHT,
+    FONT_SIZE_STEP,
+    FONT_WEIGHT_STEP,
+    LETTER_SPACING_STEP,
+    LINE_HEIGHT_STEP,
+    MAX_FONT_SIZE,
+    MAX_FONT_WEIGHT,
+    MAX_LETTER_SPACING,
+    MAX_LINE_HEIGHT,
+    MIN_FONT_SIZE,
+    MIN_FONT_WEIGHT,
+    MIN_LETTER_SPACING,
+    MIN_LINE_HEIGHT,
+    VIRTUAL_INDEX_NOT_LOADED,
+} from './const/const';
 
-export { getFontCatalog } from './store';
+// Stores (lazy accessors + classes)
+export {
+    __resetFontLifecycleManager,
+    FontLifecycleManager,
+    FontsByIdsStore,
+    getFontCatalog,
+    getFontLifecycleManager,
+} from './store';
+export type { FontCatalogStore } from './store';
 
-export * from './store';
-export * from './types';
+export type {
+    FilterGroup,
+    FilterType,
+    FontCategory,
+    FontCollectionFilters,
+    FontCollectionSort,
+    FontCollectionState,
+    FontFeatures,
+    FontFilters,
+    FontLoadRequestConfig,
+    FontLoadStatus,
+    FontMetadata,
+    FontProvider,
+    FontStyleUrls,
+    FontSubset,
+    FontVariant,
+    FontWeight,
+    FontWeightItalic,
+    UnifiedFont,
+    UnifiedFontVariant,
+} from './types';

src/entities/Font/model/store/fontCatalogStore/fontCatalogStore.svelte.spec.ts

@@ -27,18 +27,21 @@ vi.mock('$shared/api/queryClient', async importOriginal => {
      */
     const { QueryClient } = await import('@tanstack/query-core');
     const actual = await importOriginal<typeof import('$shared/api/queryClient')>();
+    const mockClient = new QueryClient({
+        defaultOptions: { queries: { retry: 0, gcTime: 0 } },
+    });
     return {
         ...actual,
-        queryClient: new QueryClient({
-            defaultOptions: { queries: { retry: 0, gcTime: 0 } },
-        }),
+        getQueryClient: () => mockClient,
     };
 });
 vi.mock('../../../api', () => ({ fetchProxyFonts: vi.fn() }));
 
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
 import { fetchProxyFonts } from '../../../api';
 
+const queryClient = getQueryClient();
+
 const fetch = fetchProxyFonts as ReturnType<typeof vi.fn>;
 
 type FontPage = { fonts: UnifiedFont[]; total: number; limit: number; offset: number };

src/entities/Font/model/store/fontCatalogStore/fontCatalogStore.svelte.ts

@@ -1,8 +1,9 @@
 import {
     DEFAULT_QUERY_GC_TIME_MS,
     DEFAULT_QUERY_STALE_TIME_MS,
-    queryClient,
+    getQueryClient,
 } from '$shared/api/queryClient';
+import { createSingleton } from '$shared/lib/helpers/createSingleton/createSingleton';
 import {
     type InfiniteData,
     InfiniteQueryObserver,
@@ -46,7 +47,7 @@ export class FontCatalogStore {
         readonly unknown[],
         PageParam
     >;
-    #qc = queryClient;
+    #qc = getQueryClient();
     #unsubscribe: () => void;
 
     constructor(params: FontStoreParams = {}) {
@@ -483,14 +484,12 @@ export class FontCatalogStore {
     }
 }
 
-let _catalog: FontCatalogStore | undefined;
+const catalog = createSingleton(
+    () => new FontCatalogStore({ limit: 50 }),
+    instance => instance.destroy(),
+);
 
-export function getFontCatalog(): FontCatalogStore {
-    return (_catalog ??= new FontCatalogStore({ limit: 50 }));
-}
+export const getFontCatalog = catalog.get;
 
 // test-only reset, so specs don't share a live observer
-export function __resetFontCatalog() {
-    _catalog?.destroy();
-    _catalog = undefined;
-}
+export const __resetFontCatalog = catalog.reset;

src/entities/Font/model/store/fontLifecycleManager/fontLifecycleManager.svelte.ts

@@ -1,3 +1,4 @@
+import { createSingleton } from '$shared/lib/helpers/createSingleton/createSingleton';
 import { SvelteMap } from 'svelte/reactivity';
 import {
     type FontLoadRequestConfig,
@@ -419,18 +420,16 @@ export class FontLifecycleManager {
     }
 }
 
-let _fontLifecycleManager: FontLifecycleManager | undefined;
-
 /**
  * App-wide font lifecycle manager, created on first access. Lazy so its
  * AbortController / FontFace bookkeeping isn't set up at module load.
  */
-export function getFontLifecycleManager(): FontLifecycleManager {
-    return (_fontLifecycleManager ??= new FontLifecycleManager());
-}
+const fontLifecycleManager = createSingleton(
+    () => new FontLifecycleManager(),
+    instance => instance.destroy(),
+);
+
+export const getFontLifecycleManager = fontLifecycleManager.get;
 
 // test-only reset, so specs don't share loaded-font/eviction state
-export function __resetFontLifecycleManager() {
-    _fontLifecycleManager?.destroy();
-    _fontLifecycleManager = undefined;
-}
+export const __resetFontLifecycleManager = fontLifecycleManager.reset;

src/entities/Font/model/store/fontLifecycleManager/utils/loadFont/loadFont.test.ts

@@ -71,7 +71,7 @@ describe('loadFont', () => {
     it('throws FontParseError when font.load() rejects', async () => {
         const loadError = new Error('parse failed');
         const MockFontFace = vi.fn(
-            function(this: any, name: string, buffer: BufferSource, options: FontFaceDescriptors) {
+            function(this: any, _name: string, _buffer: BufferSource, _options: FontFaceDescriptors) {
                 this.load = vi.fn().mockRejectedValue(loadError);
             },
         );

src/entities/Font/model/store/fontsByIdsStore/fontsByIdsStore.test.ts

@@ -1,4 +1,6 @@
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
+
+const queryClient = getQueryClient();
 import { fontKeys } from '$shared/api/queryKeys';
 import {
     beforeEach,

src/entities/Font/model/store/index.ts

@@ -1,9 +1,12 @@
 // Font lifecycle manager (browser-side load + cache + eviction)
-export * from './fontLifecycleManager/fontLifecycleManager.svelte';
+export {
+    __resetFontLifecycleManager,
+    FontLifecycleManager,
+    getFontLifecycleManager,
+} from './fontLifecycleManager/fontLifecycleManager.svelte';
 
 // Paginated catalog
 export { getFontCatalog } from './fontCatalogStore/fontCatalogStore.svelte';
-
 export type { FontCatalogStore } from './fontCatalogStore/fontCatalogStore.svelte';
 
 // Batch fetch by IDs (detail-cache seeding)

src/entities/Font/model/types/index.ts

@@ -23,4 +23,7 @@ export type {
     FontCollectionState,
 } from './store';
 
-export * from './store/fontLifecycle';
+export type {
+    FontLoadRequestConfig,
+    FontLoadStatus,
+} from './store/fontLifecycle';

src/entities/Font/testing/fonts.mock.ts

@@ -1,9 +1,5 @@
 /**
- * ============================================================================
- * MOCK FONT DATA
- * ============================================================================
- *
- * Factory functions and preset mock data for fonts.
+ * Mock font data: factory functions and preset fixtures.
  * Used in Storybook stories, tests, and development.
  *
  * ## Usage

src/entities/Font/testing/index.ts

@@ -1,8 +1,5 @@
 /**
- * ============================================================================
- * MOCK DATA HELPERS - MAIN EXPORT
- * ============================================================================
- *
+ * Mock data helpers (main export).
  * Comprehensive mock data for Storybook stories, tests, and development.
  *
  * ## Quick Start

src/entities/Font/testing/stores.mock.ts

@@ -21,11 +21,7 @@
  */
 
 import type { UnifiedFont } from '$entities/Font/model/types';
-import type {
-    QueryKey,
-    QueryObserverResult,
-    QueryStatus,
-} from '@tanstack/svelte-query';
+import type { QueryStatus } from '@tanstack/svelte-query';
 import {
     UNIFIED_FONTS,
     generateMockFonts,

src/entities/Font/ui/FontSampler/FontSampler.stories.svelte

@@ -4,7 +4,7 @@ import { defineMeta } from '@storybook/addon-svelte-csf';
 import FontSampler from './FontSampler.svelte';
 
 const { Story } = defineMeta({
-    title: 'Features/FontSampler',
+    title: 'Entities/Font/FontSampler',
     component: FontSampler,
     tags: ['autodocs'],
     parameters: {
@@ -39,8 +39,8 @@ const { Story } = defineMeta({
 </script>
 
 <script lang="ts">
-import type { UnifiedFont } from '$entities/Font';
 import type { ComponentProps } from 'svelte';
+import type { UnifiedFont } from '../../model/types';
 
 // Mock fonts for testing
 const mockArial: UnifiedFont = {
@@ -84,6 +84,14 @@ const mockGeorgia: UnifiedFont = {
         isVariable: false,
     },
 };
+
+// Stand-in for the AdjustTypography store the composing widget injects.
+const mockTypography = {
+    renderedSize: 48,
+    weight: 400,
+    height: 1.5,
+    spacing: 0,
+};
 </script>
 
 <Story
@@ -93,6 +101,7 @@ const mockGeorgia: UnifiedFont = {
         status: 'loaded',
         text: 'The quick brown fox jumps over the lazy dog',
         index: 0,
+        typography: mockTypography,
     }}
 >
     {#snippet template(args: ComponentProps<typeof FontSampler>)}
@@ -111,6 +120,7 @@ const mockGeorgia: UnifiedFont = {
         text:
             'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris.',
         index: 1,
+        typography: mockTypography,
     }}
 >
     {#snippet template(args: ComponentProps<typeof FontSampler>)}

src/entities/Font/ui/FontSampler/FontSampler.svelte

@@ -4,12 +4,6 @@
     Visual design matches FontCard: sharp corners, red hover accent, header stats.
 -->
 <script lang="ts">
-import {
-    FontApplicator,
-    type FontLoadStatus,
-    type UnifiedFont,
-} from '$entities/Font';
-import { getTypographySettingsStore } from '$features/AdjustTypography/model';
 import {
     Badge,
     ContentEditable,
@@ -18,6 +12,35 @@ import {
     Stat,
 } from '$shared/ui';
 import { fly } from 'svelte/transition';
+import type {
+    FontLoadStatus,
+    UnifiedFont,
+} from '../../model/types';
+import FontApplicator from '../FontApplicator/FontApplicator.svelte';
+
+/**
+ * Minimal typography contract this view renders with. The AdjustTypography
+ * store satisfies it structurally; defining it here keeps the entity decoupled
+ * from that feature (no entity -> feature import).
+ */
+interface FontSampleTypography {
+    /**
+     * Rendered font size in px
+     */
+    renderedSize: number;
+    /**
+     * Numeric font weight
+     */
+    weight: number;
+    /**
+     * Line-height multiplier
+     */
+    height: number;
+    /**
+     * Letter spacing
+     */
+    spacing: number;
+}
 
 interface Props {
     /**
@@ -39,11 +62,15 @@ interface Props {
      * @default 0
      */
     index?: number;
+    /**
+     * Typography settings to render the sample with. Injected by the composing
+     * widget (which owns the AdjustTypography store) so this entity view stays
+     * decoupled from that feature — the same inversion as `status`.
+     */
+    typography: FontSampleTypography;
 }
 
-let { font, status, text = $bindable(), index = 0 }: Props = $props();
-
-const typographySettingsStore = getTypographySettingsStore();
+let { font, status, text = $bindable(), index = 0, typography }: Props = $props();
 
 // Extract provider badge with fallback
 const providerBadge = $derived(
@@ -52,10 +79,10 @@ const providerBadge = $derived(
 );
 
 const stats = $derived([
-    { label: 'SZ', value: `${typographySettingsStore.renderedSize}PX` },
-    { label: 'WGT', value: `${typographySettingsStore.weight}` },
-    { label: 'LH', value: typographySettingsStore.height?.toFixed(2) },
-    { label: 'LTR', value: `${typographySettingsStore.spacing}` },
+    { label: 'SZ', value: `${typography.renderedSize}PX` },
+    { label: 'WGT', value: `${typography.weight}` },
+    { label: 'LH', value: typography.height.toFixed(2) },
+    { label: 'LTR', value: `${typography.spacing}` },
 ]);
 </script>
 
@@ -73,9 +100,8 @@ const stats = $derived([
         min-h-60
         rounded-none
     "
-    style:font-weight={typographySettingsStore.weight}
+    style:font-weight={typography.weight}
 >
-    <!-- ── Header bar ─────────────────────────────────────────────────── -->
     <div
         class="
             flex items-center justify-between
@@ -136,19 +162,18 @@ const stats = $derived([
         </div>
     </div>
 
-    <!-- ── Main content area ──────────────────────────────────────────── -->
     <div class="flex-1 p-4 sm:p-5 md:p-8 flex items-center overflow-hidden bg-paper dark:bg-dark-card relative z-10">
         <FontApplicator {font} {status}>
             <ContentEditable
                 bind:text
-                fontSize={typographySettingsStore.renderedSize}
-                lineHeight={typographySettingsStore.height}
-                letterSpacing={typographySettingsStore.spacing}
+                fontSize={typography.renderedSize}
+                lineHeight={typography.height}
+                letterSpacing={typography.spacing}
             />
         </FontApplicator>
     </div>
 
-    <!-- ── Mobile stats footer (md:hidden — header stats take over above) -->
+    <!-- Mobile stats footer; md:hidden because the header stats take over above -->
     <div class="md:hidden px-4 sm:px-5 py-1.5 sm:py-2 border-t border-subtle flex gap-2 sm:gap-4 bg-paper dark:bg-dark-card mt-auto">
         {#each stats as stat, i}
             <Footnote class="text-5xs sm:text-4xs tracking-wider {i === 0 ? 'ml-auto' : ''}">
@@ -160,7 +185,6 @@ const stats = $derived([
         {/each}
     </div>
 
-    <!-- ── Red hover line ─────────────────────────────────────────────── -->
     <div
         class="
             absolute bottom-0 left-0 right-0

src/entities/Font/ui/index.ts

@@ -1,7 +1,9 @@
 import FontApplicator from './FontApplicator/FontApplicator.svelte';
+import FontSampler from './FontSampler/FontSampler.svelte';
 import FontVirtualList from './FontVirtualList/FontVirtualList.svelte';
 
 export {
     FontApplicator,
+    FontSampler,
     FontVirtualList,
 };

src/entities/Pairing/domain/comboKey/comboKey.test.ts

@@ -0,0 +1,20 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { comboKey } from './comboKey';
+
+describe('comboKey', () => {
+    it('derives a key from the two font ids', () => {
+        expect(comboKey({ id: 'x', headerFontId: 'Inter', bodyFontId: 'Lora' })).toBe('Inter|Lora');
+    });
+    it('ignores the surrogate id (content not identity)', () => {
+        const a = comboKey({ id: 'a', headerFontId: 'Inter', bodyFontId: 'Lora' });
+        const b = comboKey({ id: 'b', headerFontId: 'Inter', bodyFontId: 'Lora' });
+        expect(a).toBe(b);
+    });
+    it('is order-sensitive on role', () => {
+        expect(comboKey({ id: 'x', headerFontId: 'Lora', bodyFontId: 'Inter' })).toBe('Lora|Inter');
+    });
+});

src/entities/Pairing/domain/comboKey/comboKey.ts

@@ -0,0 +1,13 @@
+import type { Pairing } from '../types';
+
+/**
+ * Natural key describing a Pairing's current fonts (not its identity).
+ * Used for URL share-encoding and "is this combo already on the board" checks.
+ * Recomputed on swap; two cards may share a comboKey but never an id.
+ *
+ * @param pairing - The pairing whose fonts form the key (its `id` is ignored).
+ * @returns The `headerFontId|bodyFontId` key.
+ */
+export function comboKey(pairing: Pairing): string {
+    return `${pairing.headerFontId}|${pairing.bodyFontId}`;
+}

src/entities/Pairing/domain/createPairing/createPairing.test.ts

@@ -0,0 +1,22 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { createPairing } from './createPairing';
+
+describe('createPairing', () => {
+    it('builds a pairing from two font ids', () => {
+        const p = createPairing('Inter', 'Lora');
+        expect(p.headerFontId).toBe('Inter');
+        expect(p.bodyFontId).toBe('Lora');
+    });
+    it('generates a unique id each call (duplicates stay distinct)', () => {
+        const a = createPairing('Inter', 'Lora');
+        const b = createPairing('Inter', 'Lora');
+        expect(a.id).not.toBe(b.id);
+    });
+    it('accepts an explicit id for rehydration', () => {
+        expect(createPairing('Inter', 'Lora', 'fixed-id').id).toBe('fixed-id');
+    });
+});

src/entities/Pairing/domain/createPairing/createPairing.ts

@@ -0,0 +1,15 @@
+import type { Pairing } from '../types';
+
+/**
+ * Creates a Pairing with a fresh surrogate id (or a supplied one when
+ * rehydrating from storage). The id is identity, never content — two pairings
+ * with the same fonts are still distinct cards.
+ *
+ * @param headerFontId - Font entity id for the header role.
+ * @param bodyFontId - Font entity id for the body role.
+ * @param id - Explicit id for rehydration; defaults to a fresh UUID.
+ * @returns The new Pairing.
+ */
+export function createPairing(headerFontId: string, bodyFontId: string, id: string = crypto.randomUUID()): Pairing {
+    return { id, headerFontId, bodyFontId };
+}

src/entities/Pairing/domain/index.ts

@@ -0,0 +1,3 @@
+export { comboKey } from './comboKey/comboKey';
+export { createPairing } from './createPairing/createPairing';
+export { nextFocalId } from './nextFocalId/nextFocalId';

src/entities/Pairing/domain/nextFocalId/nextFocalId.test.ts

@@ -0,0 +1,32 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { nextFocalId } from './nextFocalId';
+
+const ids = ['a', 'b', 'c'];
+
+describe('nextFocalId', () => {
+    it('steps forward', () => {
+        expect(nextFocalId(ids, 'a', 1)).toBe('b');
+    });
+    it('steps backward', () => {
+        expect(nextFocalId(ids, 'b', -1)).toBe('a');
+    });
+    it('wraps forward at the end', () => {
+        expect(nextFocalId(ids, 'c', 1)).toBe('a');
+    });
+    it('wraps backward at the start', () => {
+        expect(nextFocalId(ids, 'a', -1)).toBe('c');
+    });
+    it('returns the only id when list has one', () => {
+        expect(nextFocalId(['solo'], 'solo', 1)).toBe('solo');
+    });
+    it('returns current when focal id is absent', () => {
+        expect(nextFocalId(ids, 'missing', 1)).toBe('missing');
+    });
+    it('returns null for an empty list', () => {
+        expect(nextFocalId([], 'x', 1)).toBeNull();
+    });
+});

src/entities/Pairing/domain/nextFocalId/nextFocalId.ts

@@ -0,0 +1,21 @@
+/**
+ * The id one step from `currentId` in board order, wrapping at both ends.
+ *
+ * @param orderedIds - Pairing ids in board order.
+ * @param currentId - The currently focal id to step from.
+ * @param direction - +1 for next, -1 for previous.
+ * @returns The neighbouring id (wrapped), `currentId` unchanged if it isn't in
+ * the list, or null for an empty list.
+ */
+export function nextFocalId(orderedIds: string[], currentId: string, direction: 1 | -1): string | null {
+    if (orderedIds.length === 0) {
+        return null;
+    }
+    const i = orderedIds.indexOf(currentId);
+    if (i === -1) {
+        return currentId;
+    }
+    const len = orderedIds.length;
+    const next = (i + direction + len) % len;
+    return orderedIds[next];
+}

src/entities/Pairing/domain/types/index.ts

@@ -0,0 +1,4 @@
+export type {
+    Pairing,
+    Role,
+} from './pairing';

src/entities/Pairing/domain/types/pairing.ts

@@ -0,0 +1,26 @@
+/**
+ * A slot within a Pairing that a font fills.
+ */
+export type Role = 'header' | 'body';
+
+/**
+ * The atomic unit of comparison: a header font + a body font.
+ * Carries a surrogate `id` (stable for the card's life, never tracks content)
+ * and the two font ids it pairs. Text and typography are global to the Board,
+ * not stored here.
+ */
+export interface Pairing {
+    /**
+     * Surrogate key generated at creation, stable for the card's life.
+     * Distinguishes duplicates with identical fonts. Focal/cycling key on this.
+     */
+    id: string;
+    /**
+     * Font entity id filling the header role.
+     */
+    headerFontId: string;
+    /**
+     * Font entity id filling the body role.
+     */
+    bodyFontId: string;
+}

src/entities/Pairing/index.ts

@@ -0,0 +1,9 @@
+export {
+    comboKey,
+    createPairing,
+    nextFocalId,
+} from './domain';
+export type {
+    Pairing,
+    Role,
+} from './model/types';

src/entities/Pairing/model/types/index.ts

@@ -0,0 +1,4 @@
+export type {
+    Pairing,
+    Role,
+} from './pairing';

src/entities/Pairing/model/types/pairing.ts

@@ -0,0 +1,9 @@
+/**
+ * Re-export of the Pairing identity types. The source of truth lives in
+ * `domain/types` so the pure domain segment can reference them without importing
+ * `model` (FSD+ domain isolation: ui -> model -> domain, never back).
+ */
+export type {
+    Pairing,
+    Role,
+} from '../../domain/types';

src/features/AdjustTypography/model/store/typographySettingsStore/typographySettingsStore.svelte.ts

@@ -15,10 +15,14 @@ import {
     DEFAULT_FONT_WEIGHT,
     DEFAULT_LETTER_SPACING,
     DEFAULT_LINE_HEIGHT,
-} from '$entities/Font';
+    // Deep path (not the root barrel) on purpose: pulls only these pure
+    // constants, not the entity's UI/store graph (+ @tanstack) — keeps this
+    // feature store and its spec light at import. See audit D-1.
+} from '$entities/Font/model/const/const';
 import {
     type PersistentStore,
     createPersistentStore,
+    createSingleton,
 } from '$shared/lib';
 import type { NumericControl } from '$shared/ui';
 import { SvelteMap } from 'svelte/reactivity';
@@ -166,6 +170,7 @@ export class TypographySettingsStore {
      */
     destroy(): void {
         this.#disposeEffects();
+        this.#storage.destroy();
     }
 
     /**
@@ -302,9 +307,6 @@ export class TypographySettingsStore {
             if (c.id === 'font_size') {
                 c.instance.value = defaults.fontSize * this.#multiplier;
             } else {
-                // Map storage key to control id
-                const key = c.id.replace('_', '') as keyof TypographySettings;
-                // Simplified for brevity, you'd map these properly:
                 if (c.id === 'font_weight') {
                     c.instance.value = defaults.fontWeight;
                 }
@@ -351,22 +353,17 @@ export function createTypographySettingsStore(
 
 export type TypographySettingsStoreInstance = ReturnType<typeof createTypographySettingsStore>;
 
-let _typographySettingsStore: TypographySettingsStoreInstance | undefined;
-
 /**
  * App-wide typography settings store, keyed for the comparison view.
  * Created on first access so its persistent-store sync effects aren't set up
  * at module load.
  */
-export function getTypographySettingsStore(): TypographySettingsStoreInstance {
-    return (_typographySettingsStore ??= createTypographySettingsStore(
-        DEFAULT_TYPOGRAPHY_CONTROLS_DATA,
-        COMPARISON_STORAGE_KEY,
-    ));
-}
+const typographySettingsStore = createSingleton(
+    () => createTypographySettingsStore(DEFAULT_TYPOGRAPHY_CONTROLS_DATA, COMPARISON_STORAGE_KEY),
+    instance => instance.destroy(),
+);
+
+export const getTypographySettingsStore = typographySettingsStore.get;
 
 // test-only reset, so specs don't share persisted typography state or leak effects
-export function __resetTypographySettingsStore() {
-    _typographySettingsStore?.destroy();
-    _typographySettingsStore = undefined;
-}
+export const __resetTypographySettingsStore = typographySettingsStore.reset;

src/features/AdjustTypography/model/store/typographySettingsStore/typographySettingsStore.test.ts

@@ -6,7 +6,7 @@ import {
     DEFAULT_FONT_WEIGHT,
     DEFAULT_LETTER_SPACING,
     DEFAULT_LINE_HEIGHT,
-} from '$entities/Font';
+} from '$entities/Font/model/const/const';
 import {
     beforeEach,
     describe,
@@ -51,6 +51,7 @@ describe('TypographySettingsStore - Unit Tests', () => {
     let mockPersistentStore: {
         value: TypographySettings;
         clear: () => void;
+        destroy: () => void;
     };
 
     const createMockPersistentStore = (initialValue: TypographySettings) => {
@@ -70,6 +71,7 @@ describe('TypographySettingsStore - Unit Tests', () => {
                     letterSpacing: DEFAULT_LETTER_SPACING,
                 };
             },
+            destroy() {},
         };
     };
 
@@ -535,6 +537,7 @@ describe('TypographySettingsStore - Unit Tests', () => {
                     mockStorage = v;
                 },
                 clear: clearSpy,
+                destroy() {},
             };
 
             const manager = new TypographySettingsStore(

src/features/AdjustTypography/ui/TypographyMenu/TypographyMenu.svelte

@@ -11,11 +11,11 @@ import {
     Button,
     ComboControl,
     ControlGroup,
+    Popover,
     Slider,
 } from '$shared/ui';
 import Settings2Icon from '@lucide/svelte/icons/settings-2';
 import XIcon from '@lucide/svelte/icons/x';
-import { Popover } from 'bits-ui';
 import { getContext } from 'svelte';
 import { cubicOut } from 'svelte/easing';
 import { fly } from 'svelte/transition';
@@ -74,33 +74,21 @@ $effect(() => {
 {#if !hidden}
     {#if responsive.isMobileOrTablet}
         <div class={className}>
-            <Popover.Root bind:open>
-                <Popover.Trigger>
-                    {#snippet child({ props })}
+            <Popover bind:open side="top" align="end" sideOffset={8}>
+                {#snippet trigger(props)}
                     <Button variant="primary" {...props}>
                         {#snippet icon()}
                             <Settings2Icon class="size-4" />
                         {/snippet}
                     </Button>
                 {/snippet}
-                </Popover.Trigger>
 
-                <Popover.Portal>
-                    <Popover.Content
-                        side="top"
-                        align="end"
-                        sideOffset={8}
+                {#snippet children({ close })}
+                    <div
                         class={cn(
-                            'z-50 w-72 p-4 rounded-none',
+                            'w-72 p-4 rounded-none',
                             'surface-popover',
-                            'data-[state=open]:animate-in data-[state=closed]:animate-out',
-                            'data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0',
-                            'data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95',
-                            'data-[side=top]:slide-in-from-bottom-2',
-                            'data-[side=bottom]:slide-in-from-top-2',
                         )}
-                        interactOutsideBehavior="close"
-                        escapeKeydownBehavior="close"
                     >
                         <!-- Header -->
                         <div class="flex items-center justify-between mb-3 pb-3 border-b border-subtle">
@@ -112,17 +100,13 @@ $effect(() => {
                                     CONTROLS
                                 </span>
                             </div>
-                            <Popover.Close>
-                                {#snippet child({ props })}
                             <button
-                                        {...props}
+                                onclick={close}
                                 class="flex-center size-6 rounded-none hover:bg-black/5 dark:hover:bg-white/5 transition-colors"
                                 aria-label="Close controls"
                             >
                                 <XIcon class="size-3.5 text-neutral-500" />
                             </button>
-                                {/snippet}
-                            </Popover.Close>
                         </div>
 
                         <!-- Controls -->
@@ -136,9 +120,9 @@ $effect(() => {
                                 />
                             </ControlGroup>
                         {/each}
-                    </Popover.Content>
-                </Popover.Portal>
-            </Popover.Root>
+                    </div>
+                {/snippet}
+            </Popover>
         </div>
     {:else}
         <div

src/features/Breadcrumb/model/index.ts

@@ -1,2 +1,7 @@
-export * from './store/scrollBreadcrumbsStore.svelte';
-export * from './types/types.ts';
+export {
+    __resetScrollBreadcrumbsStore,
+    createScrollBreadcrumbsStore,
+    getScrollBreadcrumbsStore,
+} from './store/scrollBreadcrumbsStore.svelte';
+export type { BreadcrumbItem } from './store/scrollBreadcrumbsStore.svelte';
+export type { NavigationAction } from './types/types.ts';

src/features/Breadcrumb/model/store/scrollBreadcrumbsStore.svelte.ts

@@ -1,3 +1,5 @@
+import { createSingleton } from '$shared/lib/helpers/createSingleton/createSingleton';
+
 /**
  * Scroll-based breadcrumb tracking store
  *
@@ -279,17 +281,15 @@ export function createScrollBreadcrumbsStore(): ScrollBreadcrumbsStore {
     return new ScrollBreadcrumbsStore();
 }
 
-let _scrollBreadcrumbsStore: ScrollBreadcrumbsStore | undefined;
-
 /**
  * App-wide scroll breadcrumbs store, created on first access.
  */
-export function getScrollBreadcrumbsStore(): ScrollBreadcrumbsStore {
-    return (_scrollBreadcrumbsStore ??= createScrollBreadcrumbsStore());
-}
+const scrollBreadcrumbsStore = createSingleton(
+    () => createScrollBreadcrumbsStore(),
+    instance => instance.destroy(),
+);
+
+export const getScrollBreadcrumbsStore = scrollBreadcrumbsStore.get;
 
 // test-only reset, so specs don't share observer/scroll state
-export function __resetScrollBreadcrumbsStore() {
-    _scrollBreadcrumbsStore?.destroy();
-    _scrollBreadcrumbsStore = undefined;
-}
+export const __resetScrollBreadcrumbsStore = scrollBreadcrumbsStore.reset;

src/features/Breadcrumb/model/store/scrollBreadcrumbsStore.test.ts

@@ -70,7 +70,6 @@ class MockIntersectionObserver implements IntersectionObserver {
 describe('ScrollBreadcrumbsStore', () => {
     let scrollListeners: Array<() => void> = [];
     let addEventListenerSpy: ReturnType<typeof vi.spyOn>;
-    let removeEventListenerSpy: ReturnType<typeof vi.spyOn>;
     let scrollToSpy: ReturnType<typeof vi.spyOn>;
 
     // Helper to create mock elements
@@ -111,7 +110,7 @@ describe('ScrollBreadcrumbsStore', () => {
 
         // Track scroll event listeners
         addEventListenerSpy = vi.spyOn(window, 'addEventListener').mockImplementation(
-            (event: string, listener: EventListenerOrEventListenerObject, options?: any) => {
+            (event: string, listener: EventListenerOrEventListenerObject, _options?: any) => {
                 if (event === 'scroll') {
                     scrollListeners.push(listener as () => void);
                 }
@@ -119,7 +118,7 @@ describe('ScrollBreadcrumbsStore', () => {
             },
         );
 
-        removeEventListenerSpy = vi.spyOn(window, 'removeEventListener').mockImplementation(
+        vi.spyOn(window, 'removeEventListener').mockImplementation(
             (event: string, listener: EventListenerOrEventListenerObject) => {
                 if (event === 'scroll') {
                     const index = scrollListeners.indexOf(listener as () => void);

src/features/Breadcrumb/ui/BreadcrumbHeader/BreadcrumbHeaderSeeded.svelte

@@ -11,10 +11,14 @@ const sections = [
     { index: 102, title: 'Spacing' },
 ];
 
-/** @type {HTMLDivElement} */
-let container;
+/** @type {HTMLDivElement | undefined} */
+let container = $state();
 
 onMount(() => {
+    if (!container) {
+        return;
+    }
+
     for (const section of sections) {
         const el = /** @type {HTMLElement} */ (container.querySelector(`[data-story-index="${section.index}"]`));
         scrollBreadcrumbsStore.add({ index: section.index, title: section.title, element: el }, 96);

src/features/ChangeAppTheme/index.ts

@@ -1,2 +1,2 @@
-export * from './model';
-export * from './ui';
+export { getThemeManager } from './model';
+export { ThemeSwitch } from './ui';

src/features/ChangeAppTheme/model/store/ThemeManager/ThemeManager.svelte.ts

@@ -28,7 +28,10 @@
  * ```
  */
 
-import { createPersistentStore } from '$shared/lib';
+import {
+    createPersistentStore,
+    createSingleton,
+} from '$shared/lib';
 
 export const STORAGE_KEY = 'glyphdiff:theme';
 
@@ -125,6 +128,7 @@ class ThemeManager {
     destroy(): void {
         this.#mediaQuery?.removeEventListener('change', this.#systemChangeHandler);
         this.#mediaQuery = null;
+        this.#store.destroy();
     }
 
     /**
@@ -194,23 +198,18 @@ class ThemeManager {
     }
 }
 
-let _themeManager: ThemeManager | undefined;
-
 /**
  * App-wide theme manager, created on first access.
  *
  * Lazy so its persistent-store subscription isn't set up at module load.
  * Call init() on mount and destroy() on unmount (see Layout).
  */
-export function getThemeManager(): ThemeManager {
-    return (_themeManager ??= new ThemeManager());
-}
+const themeManager = createSingleton(() => new ThemeManager(), instance => instance.destroy());
+
+export const getThemeManager = themeManager.get;
 
 // test-only reset, so specs don't share persisted theme state
-export function __resetThemeManager() {
-    _themeManager?.destroy();
-    _themeManager = undefined;
-}
+export const __resetThemeManager = themeManager.reset;
 
 /**
  * ThemeManager class exported for testing purposes

src/features/CompareBoard/index.ts

@@ -0,0 +1,9 @@
+export { fitColumns } from './lib';
+export {
+    __resetBoard,
+    type BoardStore,
+    FRAME_ROLE_GAP,
+    getBoard,
+    MAX_COLUMNS,
+    type RoleTypography,
+} from './model';

src/features/CompareBoard/lib/index.ts

@@ -0,0 +1,8 @@
+export {
+    combineFrameHeight,
+    type CombineFrameHeightInput,
+    fitColumns,
+    type FitColumnsInput,
+    measureRoleHeight,
+    type RoleHeightInput,
+} from './measure';

src/features/CompareBoard/lib/measure/combineFrameHeight.test.ts

@@ -0,0 +1,19 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { combineFrameHeight } from './combineFrameHeight';
+
+describe('combineFrameHeight', () => {
+    it('sums header + gap + body block heights', () => {
+        expect(combineFrameHeight({ headerHeight: 60, bodyHeight: 200, gap: 24 })).toBe(284);
+    });
+    it('omits the gap when one block is empty (zero height)', () => {
+        expect(combineFrameHeight({ headerHeight: 0, bodyHeight: 200, gap: 24 })).toBe(200);
+        expect(combineFrameHeight({ headerHeight: 60, bodyHeight: 0, gap: 24 })).toBe(60);
+    });
+    it('is zero when both blocks are empty', () => {
+        expect(combineFrameHeight({ headerHeight: 0, bodyHeight: 0, gap: 24 })).toBe(0);
+    });
+});

src/features/CompareBoard/lib/measure/combineFrameHeight.ts

@@ -0,0 +1,30 @@
+/**
+ * Inputs for combining a frame's two role blocks into one height.
+ */
+export interface CombineFrameHeightInput {
+    /**
+     * Measured header block height in px.
+     */
+    headerHeight: number;
+    /**
+     * Measured body block height in px.
+     */
+    bodyHeight: number;
+    /**
+     * Gap in px between the header and body blocks.
+     */
+    gap: number;
+}
+
+/**
+ * Total focal-frame height: header block + gap + body block. The gap only
+ * applies when both blocks have height — an empty role (no specimen text)
+ * contributes neither height nor a dangling gap.
+ *
+ * @param input - The two block heights and the inter-block gap.
+ * @returns The combined frame height in px.
+ */
+export function combineFrameHeight({ headerHeight, bodyHeight, gap }: CombineFrameHeightInput): number {
+    const gapApplies = headerHeight > 0 && bodyHeight > 0;
+    return headerHeight + bodyHeight + (gapApplies ? gap : 0);
+}

src/features/CompareBoard/lib/measure/fitColumns.test.ts

@@ -0,0 +1,28 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { fitColumns } from './fitColumns';
+
+describe('fitColumns', () => {
+    it('packs as many honest columns as fit, gap-aware', () => {
+        // each needs 600, gap 40, available 1280 -> 1 col=600, 2 cols=1240, 3=1880
+        expect(fitColumns({ naturalWidth: 600, available: 1280, gap: 40, maxColumns: 3 })).toBe(2);
+    });
+    it('never exceeds maxColumns even with room', () => {
+        expect(fitColumns({ naturalWidth: 100, available: 5000, gap: 20, maxColumns: 3 })).toBe(3);
+    });
+    it('never returns less than 1', () => {
+        expect(fitColumns({ naturalWidth: 9000, available: 300, gap: 20, maxColumns: 3 })).toBe(1);
+    });
+    it('fits a column at the exact boundary (inclusive)', () => {
+        // 2 cols: 2*600 + 1*40 = 1240 == available -> fits
+        expect(fitColumns({ naturalWidth: 600, available: 1240, gap: 40, maxColumns: 3 })).toBe(2);
+        // one px short -> only 1
+        expect(fitColumns({ naturalWidth: 600, available: 1239, gap: 40, maxColumns: 3 })).toBe(1);
+    });
+    it('respects a maxColumns of 1 even with unlimited room', () => {
+        expect(fitColumns({ naturalWidth: 100, available: 5000, gap: 20, maxColumns: 1 })).toBe(1);
+    });
+});

src/features/CompareBoard/lib/measure/fitColumns.ts

@@ -0,0 +1,41 @@
+/**
+ * Inputs for column gating.
+ */
+export interface FitColumnsInput {
+    /**
+     * The widest pairing's Pretext natural (shrink-wrap) width in px.
+     */
+    naturalWidth: number;
+    /**
+     * Total available width in px for the columns row.
+     */
+    available: number;
+    /**
+     * Gap in px between columns.
+     */
+    gap: number;
+    /**
+     * Hard cap on columns that still preserve an honest measure (2–3).
+     */
+    maxColumns: number;
+}
+
+/**
+ * How many equal honest columns fit. Uses the real per-pairing required width
+ * (Pretext shrink-wrap) — the 45–75ch rule is only a fallback bound elsewhere.
+ * `n` columns occupy `n*naturalWidth + (n-1)*gap`. Clamped to [1, maxColumns].
+ *
+ * @param input - Natural width, available width, gap, and column cap.
+ * @returns The number of columns that fit, in [1, maxColumns].
+ */
+export function fitColumns({ naturalWidth, available, gap, maxColumns }: FitColumnsInput): number {
+    let fit = 1;
+    for (let n = 2; n <= maxColumns; n++) {
+        if (n * naturalWidth + (n - 1) * gap <= available) {
+            fit = n;
+        } else {
+            break;
+        }
+    }
+    return fit;
+}

src/features/CompareBoard/lib/measure/index.ts

@@ -0,0 +1,12 @@
+export {
+    combineFrameHeight,
+    type CombineFrameHeightInput,
+} from './combineFrameHeight';
+export {
+    fitColumns,
+    type FitColumnsInput,
+} from './fitColumns';
+export {
+    measureRoleHeight,
+    type RoleHeightInput,
+} from './measureFrameHeight';

src/features/CompareBoard/lib/measure/measureFrameHeight.test.ts

@@ -0,0 +1,32 @@
+import {
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+import { measureRoleHeight } from './measureFrameHeight';
+
+describe('measureRoleHeight', () => {
+    it('multiplies pretext line count by sizePx*lineHeight', () => {
+        const layout = vi.fn().mockReturnValue({ lineCount: 3, height: 0 });
+        const prepared = {} as never;
+        // 3 lines * 20px * 1.5 = 90
+        expect(measureRoleHeight({ prepared, maxWidth: 600, sizePx: 20, lineHeight: 1.5 }, layout)).toBe(90);
+    });
+    it('passes width and pixel line-height into pretext layout', () => {
+        const layout = vi.fn().mockReturnValue({ lineCount: 1, height: 0 });
+        measureRoleHeight({ prepared: {} as never, maxWidth: 600, sizePx: 16, lineHeight: 1.25 }, layout);
+        expect(layout).toHaveBeenCalledWith(expect.anything(), 600, 16 * 1.25);
+    });
+    it('returns 0 when the text lays out to zero lines (empty specimen)', () => {
+        const layout = vi.fn().mockReturnValue({ lineCount: 0, height: 0 });
+        expect(measureRoleHeight({ prepared: {} as never, maxWidth: 600, sizePx: 16, lineHeight: 1.5 }, layout))
+            .toBe(0);
+    });
+    it('handles fractional sizes and line-heights without rounding', () => {
+        const layout = vi.fn().mockReturnValue({ lineCount: 2, height: 0 });
+        // 2 * 15.5 * 1.4 = 43.4
+        expect(measureRoleHeight({ prepared: {} as never, maxWidth: 320, sizePx: 15.5, lineHeight: 1.4 }, layout))
+            .toBeCloseTo(43.4);
+    });
+});

src/features/CompareBoard/lib/measure/measureFrameHeight.ts

@@ -0,0 +1,46 @@
+import {
+    type PreparedText,
+    layout as pretextLayout,
+} from '@chenglou/pretext';
+
+/**
+ * Inputs for measuring one role block's rendered height.
+ */
+export interface RoleHeightInput {
+    /**
+     * Pretext-prepared specimen text for this role+font.
+     */
+    prepared: PreparedText;
+    /**
+     * Available width in px (the focal frame's content width).
+     */
+    maxWidth: number;
+    /**
+     * Resolved font-size in px.
+     */
+    sizePx: number;
+    /**
+     * Unitless line-height multiplier.
+     */
+    lineHeight: number;
+}
+
+/**
+ * Height in px of a role's text block at the given width, from Pretext's
+ * pure-arithmetic line count.
+ *
+ * Height is `lineCount * sizePx * lineHeight` rather than Pretext's own
+ * `height` so it tracks the CSS box model exactly (line-height as a multiple of
+ * font-size), keeping measurement and render in lockstep — the zero-shift
+ * invariant.
+ *
+ * @param input - Prepared text plus width and resolved type metrics.
+ * @param layout - Pretext layout fn; injectable for unit tests, defaults to
+ * `@chenglou/pretext`'s `layout`.
+ * @returns The block height in px.
+ */
+export function measureRoleHeight(input: RoleHeightInput, layout = pretextLayout): number {
+    const { prepared, maxWidth, sizePx, lineHeight } = input;
+    const { lineCount } = layout(prepared, maxWidth, sizePx * lineHeight);
+    return lineCount * sizePx * lineHeight;
+}

src/features/CompareBoard/model/const/const.ts

@@ -0,0 +1,42 @@
+/**
+ * localStorage key for the persisted board (pairings + focal + specimen).
+ */
+export const BOARD_STORAGE_KEY = 'glyphdiff:board';
+
+/**
+ * Per-role typography storage key — header AdjustTypography instance.
+ */
+export const HEADER_TYPO_KEY = 'glyphdiff:typo:header';
+
+/**
+ * Per-role typography storage key — body AdjustTypography instance.
+ */
+export const BODY_TYPO_KEY = 'glyphdiff:typo:body';
+
+/**
+ * Schema version stamped into persisted board state (gates future
+ * migrations / the URL share-state codec).
+ */
+export const BOARD_SCHEMA_VERSION = 1;
+
+/**
+ * Hard cap on side-by-side columns that still preserve an honest measure.
+ */
+export const MAX_COLUMNS = 3;
+
+/**
+ * Vertical gap in px between the header block and the body block within a frame.
+ * Used by frame-height measurement so the reserved height matches the rendered
+ * layout exactly (zero-shift).
+ */
+export const FRAME_ROLE_GAP = 24;
+
+/**
+ * Default shared specimen — one header line + one body paragraph (single
+ * language). Used to seed the board and as the share-state fallback.
+ */
+export const DEFAULT_SPECIMEN = {
+    header: 'The Art of Harmonious Type',
+    body:
+        'Good typography is invisible. It guides the eye without calling attention to itself, balancing rhythm, contrast, and proportion so the reader forgets there is a typeface at all and simply reads.',
+};

src/features/CompareBoard/model/index.ts

@@ -0,0 +1,10 @@
+export {
+    FRAME_ROLE_GAP,
+    MAX_COLUMNS,
+} from './const/const';
+export {
+    __resetBoard,
+    type BoardStore,
+    getBoard,
+    type RoleTypography,
+} from './store/boardStore/boardStore.svelte';

src/features/CompareBoard/model/store/boardStore/boardStore.svelte.test.ts

@@ -0,0 +1,223 @@
+import {
+    afterEach,
+    beforeEach,
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+
+/**
+ * Font orchestration is exercised in e2e (needs a real browser/queryClient).
+ * Here we stub the entity's font stores so the board's pure logic stays testable
+ * off the network — only `candidateFontIds` derivation is asserted at this level.
+ */
+const mockLifecycle = vi.hoisted(() => ({
+    touch: vi.fn(),
+    pin: vi.fn(),
+    unpin: vi.fn(),
+}));
+
+/**
+ * Catalog stub with four fonts so the seeding effect has material to pair.
+ * Seeding only fires when storage is empty AND nothing has been added yet, so
+ * the empty/add tests (which never flush before asserting) are unaffected.
+ */
+const mockCatalog = vi.hoisted(() => ({
+    fonts: [
+        { id: 'c0', name: 'C0' },
+        { id: 'c1', name: 'C1' },
+        { id: 'c2', name: 'C2' },
+        { id: 'c3', name: 'C3' },
+    ],
+}));
+
+/** Mutable resolved-font list the stubbed FontsByIdsStore returns; reset per test. */
+const mockFonts = vi.hoisted(() => [] as { id: string; name: string }[]);
+
+vi.mock('$entities/Font', async importOriginal => {
+    const actual = await importOriginal<typeof import('$entities/Font')>();
+    class MockFontsByIdsStore {
+        setIds() {}
+        get fonts() {
+            return mockFonts;
+        }
+        get isLoading() {
+            return false;
+        }
+        destroy() {}
+    }
+    return {
+        ...actual,
+        FontsByIdsStore: MockFontsByIdsStore,
+        getFontLifecycleManager: () => mockLifecycle,
+        getFontCatalog: () => mockCatalog,
+        getFontUrl: () => 'https://example.com/font.woff2',
+    };
+});
+
+// ensureCanvasFonts needs a real browser canvas; stub it to resolve immediately.
+// Spread actual so createPersistentStore/getPretextFontString stay real.
+vi.mock('$shared/lib', async importOriginal => {
+    const actual = await importOriginal<typeof import('$shared/lib')>();
+    return { ...actual, ensureCanvasFonts: vi.fn(() => Promise.resolve()) };
+});
+
+// Pretext measures via canvas (degenerate in jsdom); stub for deterministic lines.
+vi.mock('@chenglou/pretext', () => ({
+    prepareWithSegments: vi.fn(() => ({})),
+    layout: vi.fn(() => ({ lineCount: 2, height: 0 })),
+}));
+
+import { flushSync } from 'svelte';
+import {
+    __resetBoard,
+    getBoard,
+} from './boardStore.svelte';
+
+beforeEach(() => {
+    localStorage.clear();
+    mockFonts.length = 0;
+    __resetBoard();
+});
+afterEach(() => __resetBoard());
+
+describe('boardStore', () => {
+    it('starts empty with no focal', () => {
+        const board = getBoard();
+        expect(board.pairings).toEqual([]);
+        expect(board.focalId).toBeNull();
+    });
+
+    it('adds a pairing and makes the first one focal', () => {
+        const board = getBoard();
+        const p = board.addPairing('Inter', 'Lora');
+        expect(board.pairings).toHaveLength(1);
+        expect(board.focalId).toBe(p.id);
+        expect(board.focal).toEqual(p);
+    });
+
+    it('cycles focal forward with wrap', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        const b = board.addPairing('Roboto', 'Merriweather');
+        board.setFocal(a.id);
+        board.cycle(1);
+        expect(board.focalId).toBe(b.id);
+        board.cycle(1);
+        expect(board.focalId).toBe(a.id);
+    });
+
+    it('cycles focal backward with wrap', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        const b = board.addPairing('Roboto', 'Merriweather');
+        board.setFocal(a.id);
+        board.cycle(-1);
+        expect(board.focalId).toBe(b.id);
+    });
+
+    it('empties the board and clears focal when the last pairing is removed', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        board.removePairing(a.id);
+        expect(board.pairings).toEqual([]);
+        expect(board.focalId).toBeNull();
+    });
+
+    it('duplicates a pairing as a distinct card next to the source', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        const dup = board.duplicate(a.id);
+        expect(dup.id).not.toBe(a.id);
+        expect(dup.headerFontId).toBe('Inter');
+        expect(board.pairings[1].id).toBe(dup.id);
+    });
+
+    it('swaps one role on the focal pairing', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        board.swapFont(a.id, 'body', 'Merriweather');
+        expect(board.focal?.bodyFontId).toBe('Merriweather');
+    });
+
+    it('rewrites the shared specimen (global, not per-pairing)', () => {
+        const board = getBoard();
+        board.addPairing('Inter', 'Lora');
+        board.setSpecimen('header', 'New Header');
+        expect(board.specimen.header).toBe('New Header');
+    });
+
+    it('keeps a focal when the focal pairing is removed', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        const b = board.addPairing('Roboto', 'Merriweather');
+        board.setFocal(a.id);
+        board.removePairing(a.id);
+        expect(board.pairings).toHaveLength(1);
+        expect(board.focalId).toBe(b.id);
+    });
+
+    it('seeds curated pairings from the catalog when storage is empty', () => {
+        const board = getBoard();
+        flushSync(); // let the seed effect run
+        expect(board.pairings.length).toBeGreaterThan(0);
+        expect(board.focalId).not.toBeNull();
+    });
+
+    it('does not seed when storage already has pairings', () => {
+        // pre-seed storage so a fresh board rehydrates instead of seeding
+        const first = getBoard();
+        const p = first.addPairing('Inter', 'Lora');
+        __resetBoard();
+        const restored = getBoard();
+        flushSync();
+        expect(restored.pairings).toHaveLength(1);
+        expect(restored.pairings[0].id).toBe(p.id);
+    });
+
+    it('returns fallback 0 before warm, positive height once fonts resolve and warm', async () => {
+        mockFonts.push({ id: 'Inter', name: 'Inter' }, { id: 'Lora', name: 'Lora' });
+        const board = getBoard();
+        const p = board.addPairing('Inter', 'Lora');
+        // Cold: canvas not yet warm -> reserved fallback, never a cold measure.
+        expect(board.frameHeight(p.id, 600)).toBe(0);
+        await vi.waitFor(() => expect(board.measureReady).toBe(true));
+        expect(board.frameHeight(p.id, 600)).toBeGreaterThan(0);
+    });
+
+    it('collects every distinct candidate font id for preloading', () => {
+        const board = getBoard();
+        board.addPairing('Inter', 'Lora');
+        board.addPairing('Inter', 'Merriweather'); // Inter deduped
+        expect(new Set(board.candidateFontIds)).toEqual(new Set(['Inter', 'Lora', 'Merriweather']));
+    });
+
+    it('exposes default per-role typography', () => {
+        const board = getBoard();
+        expect(board.typo.header.size).toBeGreaterThan(0);
+        expect(board.typo.header.weight).toBeGreaterThan(0);
+        expect(board.typo.body.leading).toBeGreaterThan(0);
+    });
+
+    it('sets one role typography independently via setTypo', () => {
+        const board = getBoard();
+        board.setTypo('header', { size: 64, weight: 700, leading: 1.1, tracking: -0.02 });
+        expect(board.typo.header.size).toBe(64);
+        expect(board.typo.header.weight).toBe(700);
+        // body untouched
+        expect(board.typo.body.size).not.toBe(64);
+    });
+
+    it('persists and rehydrates pairings, focal, and specimen', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        board.setSpecimen('body', 'Persisted body');
+        __resetBoard();
+        const restored = getBoard();
+        expect(restored.pairings).toHaveLength(1);
+        expect(restored.pairings[0].id).toBe(a.id);
+        expect(restored.focalId).toBe(a.id);
+        expect(restored.specimen.body).toBe('Persisted body');
+    });
+});

src/features/CompareBoard/model/store/boardStore/boardStore.svelte.ts

@@ -0,0 +1,657 @@
+/**
+ * CompareBoard store — the board singleton.
+ *
+ * Owns the comparison board's business state: the ordered list of Pairings, the
+ * single focal pairing, and the board-global specimen text (header + body).
+ * Persists to localStorage as a compact, URL-encoding-friendly blob.
+ *
+ * Typography is NOT owned here as an AdjustTypography store (features can't
+ * import sibling features). Instead the board holds plain per-role typography
+ * values, fed in via `setTypo` by `widgets/Board` (dependency inversion).
+ *
+ * Font metadata is resolved + preloaded via the Font entity (candidate
+ * preloading, focal pinning). Frame heights are Pretext-measured behind a
+ * canvas warm-gate (the zero-shift core) and memoized for flicker-free cycling.
+ */
+
+import {
+    DEFAULT_FONT_SIZE,
+    DEFAULT_FONT_WEIGHT,
+    DEFAULT_LETTER_SPACING,
+    DEFAULT_LINE_HEIGHT,
+    type FontCatalogStore,
+    type FontLifecycleManager,
+    type FontLoadRequestConfig,
+    FontsByIdsStore,
+    type UnifiedFont,
+    getFontCatalog,
+    getFontLifecycleManager,
+    getFontUrl,
+} from '$entities/Font';
+import {
+    type Pairing,
+    type Role,
+    comboKey,
+    createPairing,
+    nextFocalId,
+} from '$entities/Pairing';
+import {
+    combineFrameHeight,
+    measureRoleHeight,
+} from '$features/CompareBoard/lib/measure';
+import {
+    BOARD_SCHEMA_VERSION,
+    BOARD_STORAGE_KEY,
+    DEFAULT_SPECIMEN,
+    FRAME_ROLE_GAP,
+} from '$features/CompareBoard/model/const/const';
+import {
+    createPersistentStore,
+    createSingleton,
+    ensureCanvasFonts,
+    getPretextFontString,
+} from '$shared/lib';
+import { prepareWithSegments } from '@chenglou/pretext';
+import {
+    flushSync,
+    untrack,
+} from 'svelte';
+import { SvelteSet } from 'svelte/reactivity';
+
+/**
+ * Compact persisted board shape. Font ids are abbreviated (`h`/`b`) to keep the
+ * blob small and URL-encoding-friendly; specimen text is in localStorage but is
+ * intentionally excluded from any future URL share.
+ */
+interface PersistedBoard {
+    /**
+     * Schema version (gates migrations / the future URL codec).
+     */
+    v: number;
+    /**
+     * Pairings in board order: surrogate id + the two font ids.
+     */
+    pairings: { id: string; h: string; b: string }[];
+    /**
+     * The focal pairing's id, or null when the board is empty.
+     */
+    focalId: string | null;
+    /**
+     * Board-global specimen text.
+     */
+    specimen: { header: string; body: string };
+}
+
+const emptyBoard = (): PersistedBoard => ({
+    v: BOARD_SCHEMA_VERSION,
+    pairings: [],
+    focalId: null,
+    specimen: { ...DEFAULT_SPECIMEN },
+});
+
+/**
+ * Plain per-role typography values the board renders and measures with. Mirrors
+ * the four axes an `AdjustTypography` store exposes, but as a framework-free
+ * value shape the board owns — the inversion seam (`widgets/Board` pushes the
+ * concrete store's values in via `setTypo`). Not persisted here: the
+ * AdjustTypography stores own typography persistence.
+ */
+export interface RoleTypography {
+    /**
+     * Font size in px (honest, absolute — no responsive multiplier).
+     */
+    size: number;
+    /**
+     * Numeric font weight (100–900).
+     */
+    weight: number;
+    /**
+     * Unitless line-height multiplier.
+     */
+    leading: number;
+    /**
+     * Letter spacing in px.
+     */
+    tracking: number;
+}
+
+const defaultRoleTypography = (): RoleTypography => ({
+    size: DEFAULT_FONT_SIZE,
+    weight: DEFAULT_FONT_WEIGHT,
+    leading: DEFAULT_LINE_HEIGHT,
+    tracking: DEFAULT_LETTER_SPACING,
+});
+
+/**
+ * Singleton board store. Pairings live as a reassigned `$state` array (ordered
+ * cycling needs index order); mutations reassign so Svelte tracks them and
+ * persist synchronously through the persistent store.
+ */
+export class BoardStore {
+    /**
+     * Ordered pairings on the board.
+     */
+    #pairings = $state<Pairing[]>([]);
+    /**
+     * The focal pairing's id, or null when the board is empty.
+     */
+    #focalId = $state<string | null>(null);
+    /**
+     * Board-global specimen text shared by every pairing.
+     */
+    #specimen = $state<{ header: string; body: string }>({ ...DEFAULT_SPECIMEN });
+    /**
+     * Per-role typography, fed in by the widget from the AdjustTypography stores
+     * (dependency-inversion seam). Read by font-loading and frame measurement.
+     */
+    #typo = $state<{ header: RoleTypography; body: RoleTypography }>({
+        header: defaultRoleTypography(),
+        body: defaultRoleTypography(),
+    });
+    /**
+     * localStorage-backed mirror of the board blob.
+     */
+    #storage = createPersistentStore<PersistedBoard>(BOARD_STORAGE_KEY, emptyBoard());
+    /**
+     * Batch font-metadata resolver, kept in sync with `candidateFontIds`.
+     */
+    #fontsByIds: FontsByIdsStore;
+    /**
+     * Font load/cache/eviction manager; pinned to keep on-screen fonts resident.
+     */
+    #lifecycle: FontLifecycleManager;
+    /**
+     * Paginated font catalog — source of fonts for default seeding.
+     */
+    #fontCatalog: FontCatalogStore;
+    /**
+     * One-shot guard: only seed a default board when storage was empty at
+     * construction (never re-seed after the user empties the board).
+     */
+    #shouldSeed: boolean;
+    /**
+     * Font strings whose canvas metrics are confirmed real (warm). Reactive
+     * (SvelteSet) so a completed warm re-runs height readers. Gates `prepare()`
+     * to avoid poisoning Pretext's cache with fallback widths.
+     */
+    #warmed = new SvelteSet<string>();
+    /**
+     * Font strings with an in-flight `ensureCanvasFonts` — dedupes warm requests.
+     */
+    #warming = new Set<string>();
+    /**
+     * Memoized frame heights keyed by (combo, width, specimen, typography), so
+     * cycling back to a measured pairing is O(1) and never reflows.
+     */
+    #heightCache = new Map<string, number>();
+    /**
+     * Last computed height per pairing — the reserved fallback returned while a
+     * pairing's fonts load/warm, so the frame never collapses to 0 mid-cycle.
+     */
+    #lastHeight = new Map<string, number>();
+    /**
+     * Disposes the constructor's $effect.root. Must run on teardown.
+     */
+    #disposeEffects: () => void;
+
+    constructor() {
+        const stored = this.#storage.value;
+        this.#pairings = stored.pairings.map(p => createPairing(p.h, p.b, p.id));
+        this.#focalId = stored.focalId;
+        this.#specimen = { ...stored.specimen };
+        this.#shouldSeed = stored.pairings.length === 0;
+
+        this.#lifecycle = getFontLifecycleManager();
+        this.#fontCatalog = getFontCatalog();
+        this.#fontsByIds = new FontsByIdsStore(this.candidateFontIds);
+
+        this.#disposeEffects = $effect.root(() => {
+            // Seed a curated default board the first time the catalog is ready and
+            // storage was empty — so the screen is never blank on first visit.
+            $effect(() => {
+                if (!this.#shouldSeed || this.#pairings.length > 0) {
+                    return;
+                }
+                const fonts = this.#fontCatalog.fonts;
+                if (fonts.length < 2) {
+                    return;
+                }
+                untrack(() => {
+                    this.#shouldSeed = false;
+                    const count = Math.min(4, Math.floor(fonts.length / 2));
+                    for (let i = 0; i < count; i++) {
+                        this.addPairing(fonts[i * 2].id, fonts[i * 2 + 1].id);
+                    }
+                });
+            });
+
+            // Keep the batch query's id set in sync with the board's candidates.
+            $effect(() => {
+                this.#fontsByIds.setIds(this.candidateFontIds);
+            });
+
+            // Preload every candidate font at its role weight (brief §Performance).
+            $effect(() => {
+                const configs = this.#candidateConfigs();
+                if (configs.length > 0) {
+                    this.#lifecycle.touch(configs);
+                }
+            });
+
+            // Pin the focal pairing's fonts so eviction never drops on-screen
+            // glyphs; unpin on focal/weight change via the cleanup return.
+            $effect(() => {
+                const focal = this.focal;
+                if (!focal) {
+                    return;
+                }
+                const headerWeight = this.#typo.header.weight;
+                const bodyWeight = this.#typo.body.weight;
+                const header = this.fontById(focal.headerFontId);
+                const body = this.fontById(focal.bodyFontId);
+                if (header) {
+                    this.#lifecycle.pin(header.id, headerWeight, header.features?.isVariable);
+                }
+                if (body) {
+                    this.#lifecycle.pin(body.id, bodyWeight, body.features?.isVariable);
+                }
+                return () => {
+                    if (header) {
+                        this.#lifecycle.unpin(header.id, headerWeight, header.features?.isVariable);
+                    }
+                    if (body) {
+                        this.#lifecycle.unpin(body.id, bodyWeight, body.features?.isVariable);
+                    }
+                };
+            });
+        });
+    }
+
+    /**
+     * Builds dedup'd font-load configs for every resolvable candidate font at its
+     * role weight (header fonts at header weight, body fonts at body weight).
+     * Unresolved fonts (metadata not yet fetched) are skipped.
+     */
+    #candidateConfigs(): FontLoadRequestConfig[] {
+        const configs: FontLoadRequestConfig[] = [];
+        const seen = new Set<string>();
+        const add = (fontId: string, weight: number) => {
+            const font = this.fontById(fontId);
+            if (!font) {
+                return;
+            }
+            const url = getFontUrl(font, weight);
+            if (!url || seen.has(url)) {
+                return;
+            }
+            seen.add(url);
+            configs.push({ id: font.id, name: font.name, weight, url, isVariable: font.features?.isVariable });
+        };
+        for (const pairing of this.#pairings) {
+            add(pairing.headerFontId, this.#typo.header.weight);
+            add(pairing.bodyFontId, this.#typo.body.weight);
+        }
+        return configs;
+    }
+
+    /**
+     * Writes current state back to the persistent store. The persistent store's
+     * own effect flushes to localStorage; `destroy()` forces that flush so
+     * synchronous rehydration (and test teardown) never loses a write.
+     */
+    #persist() {
+        this.#storage.value = {
+            v: BOARD_SCHEMA_VERSION,
+            pairings: this.#pairings.map(p => ({ id: p.id, h: p.headerFontId, b: p.bodyFontId })),
+            focalId: this.#focalId,
+            specimen: { ...this.#specimen },
+        };
+    }
+
+    /**
+     * All pairings in board order (reactive).
+     */
+    get pairings(): readonly Pairing[] {
+        return this.#pairings;
+    }
+
+    /**
+     * The focal pairing's id, or null when empty (reactive).
+     */
+    get focalId(): string | null {
+        return this.#focalId;
+    }
+
+    /**
+     * The focal pairing, or undefined when empty (reactive).
+     */
+    get focal(): Pairing | undefined {
+        return this.#pairings.find(p => p.id === this.#focalId);
+    }
+
+    /**
+     * Board-global specimen text (reactive).
+     */
+    get specimen(): { header: string; body: string } {
+        return this.#specimen;
+    }
+
+    /**
+     * Per-role typography values (reactive). Fed by the widget via `setTypo`.
+     */
+    get typo(): { header: RoleTypography; body: RoleTypography } {
+        return this.#typo;
+    }
+
+    /**
+     * Replaces one role's typography values. Called by `widgets/Board` whenever
+     * the corresponding AdjustTypography store changes (the inversion seam).
+     *
+     * @param role - Which role's typography to set.
+     * @param values - The new typography values for that role.
+     */
+    setTypo(role: Role, values: RoleTypography) {
+        this.#typo = { ...this.#typo, [role]: { ...values } };
+    }
+
+    /**
+     * Every distinct font id referenced by any pairing (header or body). The
+     * preload set — kept in sync with the batch font resolver.
+     */
+    get candidateFontIds(): string[] {
+        const ids = new Set<string>();
+        for (const pairing of this.#pairings) {
+            ids.add(pairing.headerFontId);
+            ids.add(pairing.bodyFontId);
+        }
+        return [...ids];
+    }
+
+    /**
+     * Resolves a font id to its loaded metadata, or undefined if not yet fetched.
+     *
+     * @param id - Font entity id.
+     * @returns The font metadata, or undefined while loading.
+     */
+    fontById(id: string): UnifiedFont | undefined {
+        return this.#fontsByIds.fonts.find(f => f.id === id);
+    }
+
+    /**
+     * Resolves both fonts of a pairing for the UI.
+     *
+     * @param pairing - The pairing to resolve.
+     * @returns Header and body font metadata (each undefined while loading).
+     */
+    resolvePairingFonts(pairing: Pairing): { header?: UnifiedFont; body?: UnifiedFont } {
+        return {
+            header: this.fontById(pairing.headerFontId),
+            body: this.fontById(pairing.bodyFontId),
+        };
+    }
+
+    /**
+     * The focal frame's measured height at the given content width.
+     *
+     * @param contentWidth - The frame's content width in px.
+     * @returns Height in px (0 when the board is empty).
+     */
+    focalFrameHeight(contentWidth: number): number {
+        return this.#focalId ? this.frameHeight(this.#focalId, contentWidth) : 0;
+    }
+
+    /**
+     * Pre-measures a (typically next-up) pairing so cycling to it never reflows.
+     *
+     * @param pairingId - The pairing to measure ahead of time.
+     * @param contentWidth - The frame's content width in px.
+     * @returns Height in px (fallback while fonts load/warm).
+     */
+    peekFrameHeight(pairingId: string, contentWidth: number): number {
+        return this.frameHeight(pairingId, contentWidth);
+    }
+
+    /**
+     * Measured height of a pairing's frame (header block + gap + body block) at a
+     * content width, via Pretext's pure line-count arithmetic. Returns the
+     * last-known height (or 0) until both fonts are resolved AND the canvas is
+     * warm — never measures cold, which would poison Pretext's width cache
+     * forever. Results are memoized per (combo, width, specimen, typography).
+     *
+     * @param pairingId - The pairing to measure.
+     * @param contentWidth - The frame's content width in px.
+     * @returns Height in px.
+     */
+    frameHeight(pairingId: string, contentWidth: number): number {
+        const pairing = this.#pairings.find(p => p.id === pairingId);
+        if (!pairing) {
+            return 0;
+        }
+        const { header, body } = this.resolvePairingFonts(pairing);
+        const fallback = this.#lastHeight.get(pairingId) ?? 0;
+        if (!header || !body) {
+            return fallback;
+        }
+
+        const headerFont = getPretextFontString(this.#typo.header.weight, this.#typo.header.size, header.name);
+        const bodyFont = getPretextFontString(this.#typo.body.weight, this.#typo.body.size, body.name);
+
+        this.#ensureWarm([headerFont, bodyFont]);
+        // SvelteSet read is reactive: a completed warm re-runs height readers.
+        if (!this.#warmed.has(headerFont) || !this.#warmed.has(bodyFont)) {
+            return fallback;
+        }
+
+        const key = `${comboKey(pairing)}|${contentWidth}|${this.#specimen.header}|${this.#specimen.body}|`
+            + this.#typoSignature();
+        const cached = this.#heightCache.get(key);
+        if (cached !== undefined) {
+            this.#lastHeight.set(pairingId, cached);
+            return cached;
+        }
+
+        const headerHeight = measureRoleHeight({
+            prepared: prepareWithSegments(this.#specimen.header, headerFont, {
+                letterSpacing: this.#typo.header.tracking,
+            }),
+            maxWidth: contentWidth,
+            sizePx: this.#typo.header.size,
+            lineHeight: this.#typo.header.leading,
+        });
+        const bodyHeight = measureRoleHeight({
+            prepared: prepareWithSegments(this.#specimen.body, bodyFont, {
+                letterSpacing: this.#typo.body.tracking,
+            }),
+            maxWidth: contentWidth,
+            sizePx: this.#typo.body.size,
+            lineHeight: this.#typo.body.leading,
+        });
+        const height = combineFrameHeight({ headerHeight, bodyHeight, gap: FRAME_ROLE_GAP });
+        this.#heightCache.set(key, height);
+        this.#lastHeight.set(pairingId, height);
+        return height;
+    }
+
+    /**
+     * True once the focal pairing's fonts are resolved and canvas-warm — the UI
+     * gates the first paint of the focal frame on this to avoid a cold-measure
+     * flash.
+     */
+    get measureReady(): boolean {
+        const focal = this.focal;
+        if (!focal) {
+            return false;
+        }
+        const { header, body } = this.resolvePairingFonts(focal);
+        if (!header || !body) {
+            return false;
+        }
+        const headerFont = getPretextFontString(this.#typo.header.weight, this.#typo.header.size, header.name);
+        const bodyFont = getPretextFontString(this.#typo.body.weight, this.#typo.body.size, body.name);
+        return this.#warmed.has(headerFont) && this.#warmed.has(bodyFont);
+    }
+
+    /**
+     * Kicks off canvas warming for any cold font strings (dedup'd). Fire-and-
+     * forget: on resolution the strings join `#warmed`, re-running height readers.
+     */
+    #ensureWarm(fontStrings: string[]) {
+        const cold = fontStrings.filter(s => !this.#warmed.has(s) && !this.#warming.has(s));
+        if (cold.length === 0) {
+            return;
+        }
+        cold.forEach(s => this.#warming.add(s));
+        void ensureCanvasFonts(cold)
+            .then(() => {
+                cold.forEach(s => {
+                    this.#warming.delete(s);
+                    this.#warmed.add(s);
+                });
+            })
+            .catch(() => {
+                cold.forEach(s => this.#warming.delete(s));
+            });
+    }
+
+    /**
+     * Stable signature of both roles' typography, for the height memo key.
+     */
+    #typoSignature(): string {
+        const h = this.#typo.header;
+        const b = this.#typo.body;
+        return `${h.size},${h.weight},${h.leading},${h.tracking};${b.size},${b.weight},${b.leading},${b.tracking}`;
+    }
+
+    /**
+     * Adds a pairing to the end of the board. The first pairing added becomes
+     * focal.
+     *
+     * @param headerFontId - Font id for the header role.
+     * @param bodyFontId - Font id for the body role.
+     * @returns The created pairing.
+     */
+    addPairing(headerFontId: string, bodyFontId: string): Pairing {
+        const pairing = createPairing(headerFontId, bodyFontId);
+        this.#pairings = [...this.#pairings, pairing];
+        if (this.#focalId === null) {
+            this.#focalId = pairing.id;
+        }
+        this.#persist();
+        return pairing;
+    }
+
+    /**
+     * Clones a pairing as a distinct card inserted directly after the source, and
+     * makes the clone focal so the user can immediately swap one side.
+     *
+     * @param id - Source pairing id.
+     * @returns The new pairing.
+     */
+    duplicate(id: string): Pairing {
+        const index = this.#pairings.findIndex(p => p.id === id);
+        const source = this.#pairings[index];
+        const dup = createPairing(source.headerFontId, source.bodyFontId);
+        this.#pairings = [
+            ...this.#pairings.slice(0, index + 1),
+            dup,
+            ...this.#pairings.slice(index + 1),
+        ];
+        this.#focalId = dup.id;
+        this.#persist();
+        return dup;
+    }
+
+    /**
+     * Removes a pairing. If the removed pairing was focal, focal moves to a
+     * neighbour so exactly one focal always exists on a non-empty board.
+     *
+     * @param id - Pairing id to remove.
+     */
+    removePairing(id: string) {
+        let nextFocal = this.#focalId;
+        if (this.#focalId === id) {
+            // Pick a neighbour from the still-full ordered list; if the only
+            // candidate is the one being removed, the board becomes empty.
+            const candidate = nextFocalId(this.#pairings.map(p => p.id), id, 1);
+            nextFocal = candidate === id ? null : candidate;
+        }
+        this.#pairings = this.#pairings.filter(p => p.id !== id);
+        this.#focalId = nextFocal;
+        this.#persist();
+    }
+
+    /**
+     * Sets the focal pairing.
+     *
+     * @param id - Pairing id to focus.
+     */
+    setFocal(id: string) {
+        this.#focalId = id;
+        this.#persist();
+    }
+
+    /**
+     * Steps focal one pairing in board order, wrapping at both ends.
+     *
+     * @param direction - +1 for next, -1 for previous.
+     */
+    cycle(direction: 1 | -1) {
+        if (this.#focalId === null) {
+            return;
+        }
+        const next = nextFocalId(this.#pairings.map(p => p.id), this.#focalId, direction);
+        if (next !== null) {
+            this.#focalId = next;
+            this.#persist();
+        }
+    }
+
+    /**
+     * Swaps the font filling one role of a pairing.
+     *
+     * @param id - Pairing id.
+     * @param role - Which role to swap.
+     * @param fontId - New font id for that role.
+     */
+    swapFont(id: string, role: Role, fontId: string) {
+        this.#pairings = this.#pairings.map(p => {
+            if (p.id !== id) {
+                return p;
+            }
+            return role === 'header' ? { ...p, headerFontId: fontId } : { ...p, bodyFontId: fontId };
+        });
+        this.#persist();
+    }
+
+    /**
+     * Rewrites the board-global specimen for a role.
+     *
+     * @param role - Which role's text to set.
+     * @param text - New specimen text.
+     */
+    setSpecimen(role: Role, text: string) {
+        this.#specimen = { ...this.#specimen, [role]: text };
+        this.#persist();
+    }
+
+    /**
+     * Flushes the pending persist write, then disposes the persistent store.
+     * Call on teardown.
+     */
+    destroy() {
+        flushSync();
+        this.#disposeEffects();
+        this.#fontsByIds.destroy();
+        this.#storage.destroy();
+    }
+}
+
+const board = createSingleton(
+    () => new BoardStore(),
+    instance => instance.destroy(),
+);
+
+export const getBoard = board.get;
+
+// test-only reset, so specs don't share live state or persisted blobs
+export const __resetBoard = board.reset;

src/features/DisplayFont/index.ts

@@ -1 +0,0 @@
-export { FontSampler } from './ui';

src/features/DisplayFont/ui/index.ts

@@ -1,3 +0,0 @@
-import FontSampler from './FontSampler/FontSampler.svelte';
-
-export { FontSampler };

src/features/FilterAndSortFonts/api/index.ts

@@ -1 +1,6 @@
-export * from './filters/filters';
+export { fetchProxyFilters } from './filters/filters';
+export type {
+    FilterMetadata,
+    FilterOption,
+    ProxyFiltersResponse,
+} from './filters/filters';

src/features/FilterAndSortFonts/model/store/appliedFilterStore/appliedFilterStore.svelte.ts

@@ -23,7 +23,10 @@
  * ```
  */
 
-import { createFilter } from '$shared/lib';
+import {
+    createFilter,
+    createSingleton,
+} from '$shared/lib';
 import { createDebouncedState } from '$shared/lib/helpers';
 import type {
     FilterConfig,
@@ -129,8 +132,6 @@ export function createAppliedFilterStore<TValue extends string>(config: FilterCo
 
 export type AppliedFilterStore = ReturnType<typeof createAppliedFilterStore>;
 
-let _appliedFilterStore: AppliedFilterStore | undefined;
-
 /**
  * App-wide filter manager, created on first access.
  *
@@ -138,14 +139,14 @@ let _appliedFilterStore: AppliedFilterStore | undefined;
  * lives in `./bindings.svelte` and populates groups once backend filter
  * metadata arrives.
  */
-export function getAppliedFilterStore(): AppliedFilterStore {
-    return (_appliedFilterStore ??= createAppliedFilterStore<string>({
+const appliedFilterStore = createSingleton(() =>
+    createAppliedFilterStore<string>({
         queryValue: '',
         groups: [],
-    }));
-}
+    })
+);
+
+export const getAppliedFilterStore = appliedFilterStore.get;
 
 // test-only reset, so specs don't share filter/selection state
-export function __resetAppliedFilterStore() {
-    _appliedFilterStore = undefined;
-}
+export const __resetAppliedFilterStore = appliedFilterStore.reset;

src/features/FilterAndSortFonts/model/store/appliedFilterStore/appliedFilterStore.test.ts

@@ -29,12 +29,6 @@ import { createAppliedFilterStore } from './appliedFilterStore.svelte';
  * testing Svelte 5 reactive code in Node.js.
  */
 
-// Helper to flush Svelte effects (they run in microtasks)
-async function flushEffects() {
-    await Promise.resolve();
-    await Promise.resolve();
-}
-
 // Helper to create test properties
 function createTestProperties(count: number, selectedIndices: number[] = []): Property<string>[] {
     return Array.from({ length: count }, (_, i) => ({

src/features/FilterAndSortFonts/model/store/availableFilterStore/availableFilterStore.svelte.ts

@@ -20,8 +20,9 @@ import type { FilterMetadata } from '$features/FilterAndSortFonts/api/filters/fi
 import {
     DEFAULT_QUERY_GC_TIME_MS,
     DEFAULT_QUERY_STALE_TIME_MS,
-    queryClient,
+    getQueryClient,
 } from '$shared/api/queryClient';
+import { createSingleton } from '$shared/lib/helpers/createSingleton/createSingleton';
 import {
     type QueryKey,
     QueryObserver,
@@ -49,7 +50,7 @@ export class AvailableFilterStore {
     /**
      * Shared query client
      */
-    protected qc = queryClient;
+    protected qc = getQueryClient();
 
     /**
      * Creates a new filters store
@@ -126,18 +127,16 @@ export class AvailableFilterStore {
     }
 }
 
-let _availableFilterStore: AvailableFilterStore | undefined;
-
 /**
  * App-wide filter-metadata store, created on first access. Lazy so the
  * QueryObserver isn't constructed at module load.
  */
-export function getAvailableFilterStore(): AvailableFilterStore {
-    return (_availableFilterStore ??= new AvailableFilterStore());
-}
+const availableFilterStore = createSingleton(
+    () => new AvailableFilterStore(),
+    instance => instance.destroy(),
+);
+
+export const getAvailableFilterStore = availableFilterStore.get;
 
 // test-only reset, so specs don't share a live observer
-export function __resetAvailableFilterStore() {
-    _availableFilterStore?.destroy();
-    _availableFilterStore = undefined;
-}
+export const __resetAvailableFilterStore = availableFilterStore.reset;

src/features/FilterAndSortFonts/model/store/availableFilterStore/availableFilterStore.test.ts

@@ -1,4 +1,6 @@
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
+
+const queryClient = getQueryClient();
 import {
     afterEach,
     beforeEach,

src/features/FilterAndSortFonts/model/store/bindings.svelte.ts

@@ -9,7 +9,7 @@
  * observer, so it lives at module scope, not in any individual widget.
  */
 
-import { getFontCatalog } from '$entities/Font/model';
+import { getFontCatalog } from '$entities/Font';
 import { untrack } from 'svelte';
 import { mapAppliedFiltersToParams } from '../../lib/mapper/mapAppliedFiltersToParams';
 import { mapFilterMetadataToGroups } from '../../lib/mapper/mapFilterMetadataToGroups';

src/features/FilterAndSortFonts/model/store/sortStore/sortStore.svelte.ts

@@ -1,3 +1,5 @@
+import { createSingleton } from '$shared/lib/helpers/createSingleton/createSingleton';
+
 /**
  * Sort store — manages the current sort option for font listings.
  *
@@ -46,16 +48,12 @@ export function createSortStore(initial: SortOption = 'Popularity') {
 
 export type SortStore = ReturnType<typeof createSortStore>;
 
-let _sortStore: SortStore | undefined;
-
 /**
  * App-wide sort store, created on first access.
  */
-export function getSortStore(): SortStore {
-    return (_sortStore ??= createSortStore());
-}
+const sortStore = createSingleton(() => createSortStore());
+
+export const getSortStore = sortStore.get;
 
 // test-only reset, so specs don't share selection state
-export function __resetSortStore() {
-    _sortStore = undefined;
-}
+export const __resetSortStore = sortStore.reset;

src/routes/router.ts

@@ -1,6 +1,5 @@
 import { createRouter } from 'sv-router';
 import Home from './Home.svelte';
-import Redirect from './Redirect.svelte';
 
 /**
  * Single-page router for glyphdiff.
@@ -18,6 +17,8 @@ export const {
     '/': Home,
     /**
      * Any unmatched path redirects to home until additional routes exist.
+     * Lazy-loaded so `router` doesn't statically import `Redirect`, which
+     * imports `navigate` from here — breaks the import cycle.
      */
-    '*notfound': Redirect,
+    '*notfound': () => import('./Redirect.svelte'),
 });

src/shared/api/queryClient.ts

@@ -27,11 +27,16 @@ export const QUERY_RETRY_BASE_DELAY_MS = 1000;
  */
 export const QUERY_RETRY_MAX_DELAY_MS = 30000;
 
+let queryClientInstance: QueryClient | undefined;
+
 /**
- * TanStack Query client instance
+ * Shared TanStack Query client (lazy singleton).
  *
- * Configured for optimal caching and refetching behavior.
- * Used by all font stores for data fetching and caching.
+ * Construction is deferred to the first call so importing this module is inert:
+ * module eval runs no `new QueryClient()`, so the module is genuinely
+ * side-effect-free and needs no `sideEffects` allowlist exception. The
+ * app-layer `QueryProvider` is the first caller; every store reuses the same
+ * instance. Matches the lazy-accessor pattern used by the font stores.
  *
  * Cache behavior:
  * - Data stays fresh for 5 minutes (staleTime)
@@ -39,7 +44,8 @@ export const QUERY_RETRY_MAX_DELAY_MS = 30000;
  * - No refetch on window focus (reduces unnecessary network requests)
  * - 3 retries with exponential backoff on failure
  */
-export const queryClient = new QueryClient({
+export function getQueryClient(): QueryClient {
+    return (queryClientInstance ??= new QueryClient({
         defaultOptions: {
             queries: {
                 staleTime: DEFAULT_QUERY_STALE_TIME_MS,
@@ -65,4 +71,5 @@ export const queryClient = new QueryClient({
                     Math.min(QUERY_RETRY_BASE_DELAY_MS * 2 ** attemptIndex, QUERY_RETRY_MAX_DELAY_MS),
             },
         },
-});
+    }));
+}

src/shared/lib/helpers/BaseQueryStore/BaseQueryStore.svelte.ts

@@ -1,4 +1,4 @@
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
 import {
     QueryObserver,
     type QueryObserverOptions,
@@ -20,7 +20,7 @@ export abstract class BaseQueryStore<TData, TError = Error> {
     #unsubscribe: () => void;
 
     constructor(options: QueryObserverOptions<TData, TError, TData, any, any>) {
-        this.#observer = new QueryObserver(queryClient, options);
+        this.#observer = new QueryObserver(getQueryClient(), options);
         this.#unsubscribe = this.#observer.subscribe(result => {
             this.#result = result;
         });

src/shared/lib/helpers/BaseQueryStore/BaseQueryStore.test.ts

@@ -1,4 +1,6 @@
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
+
+const queryClient = getQueryClient();
 import {
     beforeEach,
     describe,

src/shared/lib/helpers/createPersistentStore/createPersistentStore.svelte.ts

@@ -1,66 +1,15 @@
 /**
- * Persistent localStorage-backed reactive state
+ * Reactive localStorage-backed state. Loads on init, saves on change via an
+ * $effect.root. Falls back to the default on SSR (no localStorage) and on JSON
+ * parse errors; swallows quota/write errors with a warning.
  *
- * Creates reactive state that automatically syncs with localStorage.
- * Values persist across browser sessions and are restored on page load.
+ * Owners that create this outside a component must call destroy() to dispose
+ * the save effect.
  *
- * Handles edge cases:
- * - SSR safety (no localStorage on server)
- * - JSON parse errors (falls back to default)
- * - Storage quota errors (logs warning, doesn't crash)
- *
- * @example
- * ```ts
- * // Store user preferences
- * const preferences = createPersistentStore('user-prefs', {
- *     theme: 'dark',
- *     fontSize: 16,
- *     sidebarOpen: true
- * });
- *
- * // Access reactive state
- * $: currentTheme = preferences.value.theme;
- *
- * // Update (auto-saves to localStorage)
- * preferences.value.theme = 'light';
- *
- * // Clear stored value
- * preferences.clear();
- * ```
- */
-
-/**
- * Creates a reactive store backed by localStorage
- *
- * The value is loaded from localStorage on initialization and automatically
- * saved whenever it changes. Uses Svelte 5's $effect for reactive sync.
- *
- * @param key - localStorage key for storing the value
- * @param defaultValue - Default value if no stored value exists
- * @returns Persistent store with getter/setter and clear method
- *
- * @example
- * ```ts
- * // Simple value
- * const counter = createPersistentStore('counter', 0);
- * counter.value++;
- *
- * // Complex object
- * interface Settings {
- *     theme: 'light' | 'dark';
- *     fontSize: number;
- * }
- * const settings = createPersistentStore<Settings>('app-settings', {
- *     theme: 'light',
- *     fontSize: 16
- * });
- * ```
+ * @param key - localStorage key
+ * @param defaultValue - value used when nothing is stored
  */
 export function createPersistentStore<T>(key: string, defaultValue: T) {
-    /**
-     * Load value from localStorage or return default
-     * Safely handles missing keys, parse errors, and SSR
-     */
     const loadFromStorage = (): T => {
         if (typeof window === 'undefined') {
             return defaultValue;
@@ -76,9 +25,13 @@ export function createPersistentStore<T>(key: string, defaultValue: T) {
 
     let value = $state<T>(loadFromStorage());
 
-    // Sync to storage whenever value changes
-    // Wrapped in $effect.root to prevent memory leaks
-    $effect.root(() => {
+    /**
+     * Sync to storage whenever value changes. The effect lives in an
+     * $effect.root so it outlives any component; the returned disposer is kept
+     * and run by destroy(), because an $effect.root with no disposer leaks for
+     * the life of the process.
+     */
+    const dispose = $effect.root(() => {
         $effect(() => {
             if (typeof window === 'undefined') {
                 return;
@@ -113,6 +66,15 @@ export function createPersistentStore<T>(key: string, defaultValue: T) {
             }
             value = defaultValue;
         },
+
+        /**
+         * Dispose the storage-sync effect. Owners that create a store outside a
+         * component (e.g. a singleton store class) must call this to avoid
+         * leaking the underlying $effect.root.
+         */
+        destroy() {
+            dispose();
+        },
     };
 }
 

src/shared/lib/helpers/createPersistentStore/createPersistentStore.test.ts

@@ -1,6 +1,7 @@
 /**
  * @vitest-environment jsdom
  */
+import { flushSync } from 'svelte';
 import {
     afterEach,
     beforeEach,
@@ -376,4 +377,39 @@ describe('createPersistentStore', () => {
             expect(store.value[0].name).toBe('First');
         });
     });
+
+    describe('Lifecycle', () => {
+        it('persists value changes via the sync effect', () => {
+            const store = createPersistentStore(testKey, 'a');
+            const spy = vi.spyOn(mockLocalStorage, 'setItem');
+
+            store.value = 'b';
+            flushSync();
+
+            expect(spy).toHaveBeenCalledWith(testKey, JSON.stringify('b'));
+        });
+
+        it('stops persisting after destroy()', () => {
+            const store = createPersistentStore(testKey, 'a');
+            flushSync();
+            store.destroy();
+
+            const spy = vi.spyOn(mockLocalStorage, 'setItem');
+            store.value = 'c';
+            flushSync();
+
+            expect(spy).not.toHaveBeenCalled();
+            // reading still works after disposal
+            expect(store.value).toBe('c');
+        });
+
+        it('destroy() is safe to call repeatedly', () => {
+            const store = createPersistentStore(testKey, 'a');
+
+            expect(() => {
+                store.destroy();
+                store.destroy();
+            }).not.toThrow();
+        });
+    });
 });

src/shared/lib/helpers/createSingleton/createSingleton.test.ts

@@ -0,0 +1,86 @@
+import {
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+import { createSingleton } from './createSingleton';
+
+describe('createSingleton', () => {
+    it('does not call the factory until the first get (lazy)', () => {
+        const factory = vi.fn(() => ({ id: 1 }));
+        createSingleton(factory);
+        expect(factory).not.toHaveBeenCalled();
+    });
+
+    it('constructs on first get and memoizes the instance', () => {
+        const factory = vi.fn(() => ({ id: 1 }));
+        const singleton = createSingleton(factory);
+
+        const a = singleton.get();
+        const b = singleton.get();
+
+        expect(factory).toHaveBeenCalledTimes(1);
+        expect(a).toBe(b);
+    });
+
+    it('rebuilds a fresh instance after reset', () => {
+        let count = 0;
+        const singleton = createSingleton(() => ({ id: ++count }));
+
+        const first = singleton.get();
+        singleton.reset();
+        const second = singleton.get();
+
+        expect(first).not.toBe(second);
+        expect(second.id).toBe(2);
+    });
+
+    it('runs teardown once, with the live instance, on reset', () => {
+        const teardown = vi.fn();
+        const singleton = createSingleton(() => ({ id: 1 }), teardown);
+
+        const instance = singleton.get();
+        singleton.reset();
+
+        expect(teardown).toHaveBeenCalledTimes(1);
+        expect(teardown).toHaveBeenCalledWith(instance);
+    });
+
+    it('treats reset before any get as a no-op (no teardown, no throw)', () => {
+        const teardown = vi.fn();
+        const singleton = createSingleton(() => ({ id: 1 }), teardown);
+
+        expect(() => singleton.reset()).not.toThrow();
+        expect(teardown).not.toHaveBeenCalled();
+    });
+
+    it('does not run teardown again on a second consecutive reset', () => {
+        const teardown = vi.fn();
+        const singleton = createSingleton(() => ({ id: 1 }), teardown);
+
+        singleton.get();
+        singleton.reset();
+        singleton.reset();
+
+        expect(teardown).toHaveBeenCalledTimes(1);
+    });
+
+    it('works without a teardown', () => {
+        const singleton = createSingleton(() => ({ id: 1 }));
+
+        singleton.get();
+        expect(() => singleton.reset()).not.toThrow();
+        expect(singleton.get().id).toBe(1);
+    });
+
+    it('caches a falsy instance value without re-running the factory', () => {
+        const factory = vi.fn(() => undefined);
+        const singleton = createSingleton<undefined>(factory);
+
+        singleton.get();
+        singleton.get();
+
+        expect(factory).toHaveBeenCalledTimes(1);
+    });
+});

src/shared/lib/helpers/createSingleton/createSingleton.ts

@@ -0,0 +1,57 @@
+/**
+ * A lazily-constructed singleton accessor pair.
+ */
+export interface Singleton<T> {
+    /**
+     * Returns the instance, constructing it on the first call and reusing it
+     * thereafter.
+     */
+    get: () => T;
+    /**
+     * Tears down the current instance (if built) and clears it, so the next
+     * `get()` rebuilds. Used by specs to avoid shared state between tests.
+     */
+    reset: () => void;
+}
+
+/**
+ * Standardizes the lazy `getX()` / `__resetX()` singleton pattern used by the
+ * app's stores.
+ *
+ * The instance is built on the first `get()` and reused afterwards; `reset()`
+ * runs the optional teardown against the live instance and clears it. Building
+ * lazily keeps the owning module inert at import — construction happens only on
+ * first access, never at module eval.
+ *
+ * @param factory - Builds the instance on first access.
+ * @param teardown - Optional cleanup run against the live instance on reset
+ * (e.g. disposing an `$effect.root` via the instance's `destroy()`).
+ *
+ * @example
+ * ```ts
+ * const catalog = createSingleton(() => new FontCatalogStore({ limit: 50 }), c => c.destroy());
+ * export const getFontCatalog = catalog.get;
+ * export const __resetFontCatalog = catalog.reset;
+ * ```
+ */
+export function createSingleton<T>(factory: () => T, teardown?: (instance: T) => void): Singleton<T> {
+    let instance: T | undefined;
+    let initialized = false;
+
+    return {
+        get: () => {
+            if (!initialized) {
+                instance = factory();
+                initialized = true;
+            }
+            return instance as T;
+        },
+        reset: () => {
+            if (initialized) {
+                teardown?.(instance as T);
+            }
+            instance = undefined;
+            initialized = false;
+        },
+    };
+}

src/shared/lib/helpers/createVirtualizer/createVirtualizer.svelte.ts

@@ -156,7 +156,7 @@ export function createVirtualizer<T>(
     const offsets = $derived.by(() => {
         const count = options.count;
         // Implicit dependency on version signal
-        const v = _version;
+        const _v = _version;
         const result = new Float64Array(count);
         let accumulated = 0;
         for (let i = 0; i < count; i++) {
@@ -180,7 +180,7 @@ export function createVirtualizer<T>(
         // this derivation when the items array is replaced!
         const { count, data } = options;
         // Implicit dependency
-        const v = _version;
+        const _v = _version;
         if (count === 0 || containerHeight === 0 || !data) {
             return [];
         }
@@ -268,7 +268,6 @@ export function createVirtualizer<T>(
                 return rect.top + scrollY;
             };
 
-            let cachedOffsetTop = 0;
             let rafId: number | null = null;
             containerHeight = typeof window !== 'undefined' ? window.innerHeight : 0;
 
@@ -292,14 +291,12 @@ export function createVirtualizer<T>(
             const handleResize = () => {
                 containerHeight = window.innerHeight;
                 elementOffsetTop = getElementOffset();
-                cachedOffsetTop = elementOffsetTop;
                 handleScroll();
             };
 
             // Initial setup
             requestAnimationFrame(() => {
                 elementOffsetTop = getElementOffset();
-                cachedOffsetTop = elementOffsetTop;
                 handleScroll();
             });
 

src/shared/lib/helpers/index.ts

@@ -137,6 +137,20 @@ export {
     type PerspectiveManager,
 } from './createPerspectiveManager/createPerspectiveManager.svelte';
 
+/**
+ * Lazy singletons
+ */
+export {
+    /**
+     * Lazy `getX()` / `__resetX()` singleton accessor factory
+     */
+    createSingleton,
+    /**
+     * Singleton accessor pair type
+     */
+    type Singleton,
+} from './createSingleton/createSingleton';
+
 /*
  * BaseQueryStore is intentionally NOT re-exported here.
  * It pulls @tanstack/query-core, so routing it through this leaf barrel would

src/shared/lib/index.ts

@@ -11,6 +11,7 @@ export {
     createPersistentStore,
     createPerspectiveManager,
     createResponsiveManager,
+    createSingleton,
     createVirtualizer,
     type Entity,
     type EntityStore,
@@ -21,6 +22,7 @@ export {
     type Property,
     type ResponsiveManager,
     responsiveManager,
+    type Singleton,
     type VirtualItem,
     type Virtualizer,
     type VirtualizerOptions,
@@ -31,7 +33,9 @@ export {
     clampNumber,
     cn,
     debounce,
+    ensureCanvasFonts,
     getDecimalPlaces,
+    getPretextFontString,
     roundToStepPrecision,
     smoothScroll,
     splitArray,

src/shared/lib/storybook/index.ts

@@ -1,9 +1,5 @@
 /**
- * ============================================================================
- * STORYBOOK HELPERS
- * ============================================================================
- *
- * Helper components and utilities for Storybook stories.
+ * Storybook helpers: components and utilities for stories.
  *
  * ## Usage
  *

src/shared/lib/transitions/springySlideFade/springySlideFade.ts

@@ -3,10 +3,6 @@ import type {
     TransitionConfig,
 } from 'svelte/transition';
 
-function elasticOut(t: number) {
-    return Math.pow(2, -10 * t) * Math.sin((t - 0.075) * (2 * Math.PI) / 0.3) + 1;
-}
-
 function gentleSpring(t: number) {
     return 1 - Math.pow(1 - t, 3) * Math.cos(t * Math.PI * 2);
 }

src/shared/lib/utils/ensureCanvasFonts/ensureCanvasFonts.ts

@@ -0,0 +1,71 @@
+/**
+ * Ensures a set of fonts is usable in a `<canvas>` measurement context.
+ *
+ * `document.fonts.load()` resolves once the FontFace bytes are fetched and
+ * parsed, but Chrome lazily registers fonts with the canvas measurement engine
+ * after that — `measureText` keeps returning a fallback width for some frames
+ * even though `document.fonts.check()` reports the font as loaded.
+ *
+ * Pretext caches measurements per font string forever, so a single fallback
+ * measurement during initial mount permanently poisons the cache and the
+ * rendered text drifts visibly from its measured box. This helper polls canvas
+ * measurement until each font reports a width that differs from the "unknown
+ * font family" fallback, guaranteeing the next `measureText` call sees the real
+ * glyph metrics.
+ *
+ * ponytail: deliberate copy of widgets/ComparisonView/lib's version — ADR-0002
+ * keeps the shelved morph tool untouched, so we don't move its util. The poll
+ * logic is the proven fix for Pretext's fallback-width cache poisoning; copying
+ * it is cheaper than refactoring frozen code.
+ *
+ * @param fontStrings - Pretext/canvas font strings (`weight sizepx "family"`) to warm.
+ */
+import { getPretextFontString } from '../getPretextFontString/getPretextFontString';
+
+const PROBE_TEXT = 'mmmmmmmmmm';
+const MAX_WAIT_MS = 1000;
+const DEFAULT_PROBE_SIZE_PX = 16;
+// Family unlikely to exist in any system — gives canvas's "unknown font" fallback width.
+const FALLBACK_PROBE_FAMILY = '__glyphdiff_no_such_font_42__';
+
+export async function ensureCanvasFonts(fontStrings: string[]): Promise<void> {
+    await Promise.all(fontStrings.map(f => document.fonts.load(f)));
+
+    // Pretext uses OffscreenCanvas when available; DOM canvas has separate font
+    // registration timing, so we MUST poll using the same canvas type pretext does.
+    const ctx = typeof OffscreenCanvas !== 'undefined'
+        ? new OffscreenCanvas(1, 1).getContext('2d')
+        : document.createElement('canvas').getContext('2d');
+    if (!ctx) {
+        return;
+    }
+
+    // Measure each font's "unknown font" fallback width (different per browser, per OS).
+    // Canvas uses this same fallback for any font family it can't resolve, so when the
+    // requested font finally registers, measureText will return a non-fallback width.
+    const fallbackWidths = new Map<string, number>();
+    for (const font of fontStrings) {
+        const sizeMatch = font.match(/(\d+(?:\.\d+)?)px/);
+        const sizePx = sizeMatch ? parseFloat(sizeMatch[1]) : DEFAULT_PROBE_SIZE_PX;
+        ctx.font = getPretextFontString(400, sizePx, FALLBACK_PROBE_FAMILY);
+        fallbackWidths.set(font, ctx.measureText(PROBE_TEXT).width);
+    }
+
+    const deadline = performance.now() + MAX_WAIT_MS;
+    const pending = new Set(fontStrings);
+    while (pending.size > 0 && performance.now() < deadline) {
+        for (const font of Array.from(pending)) {
+            ctx.font = font;
+            const w = ctx.measureText(PROBE_TEXT).width;
+            if (Math.abs(w - fallbackWidths.get(font)!) > 0.5) {
+                pending.delete(font);
+            }
+        }
+        if (pending.size === 0) {
+            break;
+        }
+        // Sequential by design: poll once per animation frame until fonts register.
+        // eslint-disable-next-line no-await-in-loop
+        await new Promise<void>(resolve => requestAnimationFrame(() => resolve()));
+    }
+}

src/shared/lib/utils/getPretextFontString/getPretextFontString.test.ts

@@ -0,0 +1,15 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { getPretextFontString } from './getPretextFontString';
+
+describe('getPretextFontString', () => {
+    it('formats weight, px size and quoted family for pretext/canvas', () => {
+        expect(getPretextFontString(400, 48, 'Inter')).toBe('400 48px "Inter"');
+    });
+    it('preserves fractional sizes and quotes multi-word family names', () => {
+        expect(getPretextFontString(700, 12.5, 'PT Serif')).toBe('700 12.5px "PT Serif"');
+    });
+});

src/shared/lib/utils/getPretextFontString/getPretextFontString.ts

@@ -0,0 +1,16 @@
+/**
+ * Formats a font config into the string `@chenglou/pretext` and the Canvas 2D
+ * `font` property both expect: `weight sizepx "family"`.
+ *
+ * ponytail: deliberate copy of widgets/ComparisonView/lib's version — ADR-0002
+ * keeps the shelved morph tool untouched, so we don't move its util. Three lines
+ * is cheaper to duplicate than to refactor frozen code.
+ *
+ * @param weight - Numeric font weight (e.g. 400).
+ * @param sizePx - Font size in pixels.
+ * @param fontName - The font family name.
+ * @returns A formatted font string: `weight sizepx "fontName"`.
+ */
+export function getPretextFontString(weight: number, sizePx: number, fontName: string): string {
+    return `${weight} ${sizePx}px "${fontName}"`;
+}

src/shared/lib/utils/index.ts

@@ -17,7 +17,9 @@ export {
 export { clampNumber } from './clampNumber/clampNumber';
 export { cn } from './cn';
 export { debounce } from './debounce/debounce';
+export { ensureCanvasFonts } from './ensureCanvasFonts/ensureCanvasFonts';
 export { getDecimalPlaces } from './getDecimalPlaces/getDecimalPlaces';
+export { getPretextFontString } from './getPretextFontString/getPretextFontString';
 export { getSkeletonWidth } from './getSkeletonWidth/getSkeletonWidth';
 export { roundToStepPrecision } from './roundToStepPrecision/roundToStepPrecision';
 export { smoothScroll } from './smoothScroll/smoothScroll';

src/shared/lib/utils/roundToStepPrecision/roundToStepPrecision.ts

@@ -1,4 +1,4 @@
-import { getDecimalPlaces } from '$shared/lib/utils';
+import { getDecimalPlaces } from '../getDecimalPlaces/getDecimalPlaces';
 
 /**
  * Rounds a value to match the precision of a given step

src/shared/lib/utils/splitArray/splitArray.ts

@@ -24,9 +24,14 @@
  */
 export function splitArray<T>(array: T[], callback: (item: T) => boolean) {
     return array.reduce<[T[], T[]]>(
-        ([pass, fail], item) => (
-            callback(item) ? pass.push(item) : fail.push(item), [pass, fail]
-        ),
+        ([pass, fail], item) => {
+            if (callback(item)) {
+                pass.push(item);
+            } else {
+                fail.push(item);
+            }
+            return [pass, fail];
+        },
         [[], []],
     );
 }

src/shared/ui/Badge/Badge.svelte

@@ -4,12 +4,12 @@
 -->
 <script lang="ts">
 import { cn } from '$shared/lib';
+import type { Snippet } from 'svelte';
+import type { HTMLAttributes } from 'svelte/elements';
 import {
     type LabelSize,
     labelSizeConfig,
-} from '$shared/ui/Label/config';
-import type { Snippet } from 'svelte';
-import type { HTMLAttributes } from 'svelte/elements';
+} from '../labelConfig';
 
 type BadgeVariant = 'default' | 'accent' | 'success' | 'warning' | 'info';
 

src/shared/ui/ComboControl/ComboControl.svelte

@@ -5,11 +5,11 @@
 -->
 <script lang="ts">
 import { cn } from '$shared/lib';
-import { Slider } from '$shared/ui';
-import { Button } from '$shared/ui/Button';
 import MinusIcon from '@lucide/svelte/icons/minus';
 import PlusIcon from '@lucide/svelte/icons/plus';
-import { Popover } from 'bits-ui';
+import { Button } from '../Button';
+import Popover from '../Popover/Popover.svelte';
+import Slider from '../Slider/Slider.svelte';
 import TechText from '../TechText/TechText.svelte';
 import type {
     ControlLabels,
@@ -84,9 +84,11 @@ const displayLabel = $derived(label ?? controlLabel ?? '');
             {formattedValue()}
         </span>
     </div>
-
-    <!-- ── FULL MODE ──────────────────────────────────────────────────────────────── -->
 {:else}
+    <!--
+        FULL MODE
+        +/- buttons flanking a slider popover.
+    -->
     <div class={cn('flex items-center px-1 relative', className)}>
         <!-- Decrease button -->
         <Button
@@ -103,9 +105,8 @@ const displayLabel = $derived(label ?? controlLabel ?? '');
 
         <!-- Trigger -->
         <div class="relative mx-1">
-            <Popover.Root bind:open>
-                <Popover.Trigger>
-                    {#snippet child({ props })}
+            <Popover bind:open side="top" align="center">
+                {#snippet trigger(props)}
                     <button
                         {...props}
                         class={cn(
@@ -138,14 +139,10 @@ const displayLabel = $derived(label ?? controlLabel ?? '');
                         </TechText>
                     </button>
                 {/snippet}
-                </Popover.Trigger>
 
                 <!-- Vertical slider popover -->
-                <Popover.Content
-                    class="w-auto py-4 px-3 h-64 flex-center rounded-none surface-card-elevated"
-                    align="center"
-                    side="top"
-                >
+                {#snippet children()}
+                    <div class="w-auto py-4 px-3 h-64 flex-center rounded-none surface-card-elevated">
                         <Slider
                             class="h-full"
                             bind:value={control.value}
@@ -154,8 +151,9 @@ const displayLabel = $derived(label ?? controlLabel ?? '');
                             step={control.step}
                             orientation="vertical"
                         />
-                </Popover.Content>
-            </Popover.Root>
+                    </div>
+                {/snippet}
+            </Popover>
         </div>
 
         <!-- Increase button -->

src/shared/ui/ComboControl/ComboControl.svelte.test.ts

@@ -3,6 +3,7 @@ import {
     render,
     screen,
     waitFor,
+    within,
 } from '@testing-library/svelte';
 import ComboControl from './ComboControl.svelte';
 import { createNumericControlMock } from './testing/createNumericControlMock.svelte';
@@ -16,6 +17,16 @@ function makeControl(value: number, opts: { min?: number; max?: number; step?: n
     });
 }
 
+/**
+ * The trigger is the button wired to the popover (has popovertarget). The native
+ * Popover always renders its content (the vertical slider, which also displays the
+ * value) in the DOM, so value assertions must be scoped to the trigger to avoid
+ * matching the slider's own value label.
+ */
+function getTrigger(): HTMLElement {
+    return document.querySelector('button[popovertarget]') as HTMLElement;
+}
+
 describe('ComboControl', () => {
     describe('Rendering', () => {
         it('renders decrease and increase buttons', () => {
@@ -26,17 +37,17 @@ describe('ComboControl', () => {
 
         it('renders the current integer value', () => {
             render(ComboControl, { control: makeControl(42) });
-            expect(screen.getByText('42')).toBeInTheDocument();
+            expect(within(getTrigger()).getByText('42')).toBeInTheDocument();
         });
 
         it('formats decimal value to 1 decimal place when step >= 0.1', () => {
             render(ComboControl, { control: makeControl(1.5, { step: 0.1 }) });
-            expect(screen.getByText('1.5')).toBeInTheDocument();
+            expect(within(getTrigger()).getByText('1.5')).toBeInTheDocument();
         });
 
         it('formats decimal value to 2 decimal places when step < 0.1', () => {
             render(ComboControl, { control: makeControl(1.55, { step: 0.01 }) });
-            expect(screen.getByText('1.55')).toBeInTheDocument();
+            expect(within(getTrigger()).getByText('1.55')).toBeInTheDocument();
         });
 
         it('renders label when label prop is provided', () => {
@@ -106,16 +117,32 @@ describe('ComboControl', () => {
             const control = makeControl(50);
             render(ComboControl, { control });
             await fireEvent.click(screen.getByLabelText('Increase'));
-            await waitFor(() => expect(screen.getByText('51')).toBeInTheDocument());
+            await waitFor(() => expect(within(getTrigger()).getByText('51')).toBeInTheDocument());
         });
     });
 
     describe('Popover', () => {
-        it('opens popover with vertical slider on trigger click', async () => {
+        /**
+         * The native Popover always renders its content; opening is driven by the
+         * browser's declarative popovertarget invoker, which jsdom does not simulate
+         * on click (mirrors Popover.svelte.test.ts). So assert the wired-but-closed
+         * state, then drive the open through the API the browser would call.
+         */
+        it('exposes a popover trigger with the vertical slider as its content', async () => {
             render(ComboControl, { control: makeControl(50), controlLabel: 'Size control' });
-            expect(screen.queryByRole('slider')).not.toBeInTheDocument();
-            await fireEvent.click(screen.getByText('Size control'));
-            await waitFor(() => expect(screen.getByRole('slider')).toBeInTheDocument());
+
+            const trigger = getTrigger();
+            expect(trigger).toHaveAttribute('aria-expanded', 'false');
+
+            const content = document.getElementById(trigger.getAttribute('popovertarget')!) as HTMLElement;
+            expect(content).toHaveAttribute('data-state', 'closed');
+            // The vertical slider lives inside the popover content. While closed the
+            // content is visibility:hidden, so query including hidden elements.
+            expect(within(content).getByRole('slider', { hidden: true })).toBeInTheDocument();
+
+            content.showPopover();
+            await waitFor(() => expect(content).toHaveAttribute('data-state', 'open'));
+            expect(trigger).toHaveAttribute('aria-expanded', 'true');
         });
     });
 

src/shared/ui/FilterGroup/FilterGroup.svelte

@@ -5,8 +5,6 @@
 <script lang="ts">
 import type { Filter } from '$shared/lib';
 import { cn } from '$shared/lib';
-import { Button } from '$shared/ui';
-import { Label } from '$shared/ui';
 import ChevronUpIcon from '@lucide/svelte/icons/chevron-up';
 import EllipsisIcon from '@lucide/svelte/icons/ellipsis';
 import { cubicOut } from 'svelte/easing';
@@ -14,6 +12,8 @@ import {
     draw,
     fly,
 } from 'svelte/transition';
+import { Button } from '../Button';
+import Label from '../Label/Label.svelte';
 
 interface Props {
     /**

src/shared/ui/Input/index.ts

@@ -1 +1,2 @@
 export { default as Input } from './Input.svelte';
+export { inputIconSize } from './types';

src/shared/ui/Label/Label.svelte

@@ -11,7 +11,7 @@ import {
     type LabelVariant,
     labelSizeConfig,
     labelVariantConfig,
-} from './config';
+} from '../labelConfig';
 
 interface Props {
     /**

src/shared/ui/Logo/Logo.svelte

@@ -4,7 +4,7 @@
 -->
 <script lang="ts">
 import { cn } from '$shared/lib';
-import { Badge } from '$shared/ui';
+import Badge from '../Badge/Badge.svelte';
 
 interface Props {
     /**

src/shared/ui/Popover/Popover.stories.svelte

@@ -0,0 +1,117 @@
+<script module>
+import { defineMeta } from '@storybook/addon-svelte-csf';
+import Popover from './Popover.svelte';
+
+const { Story } = defineMeta({
+    title: 'Shared/Popover',
+    component: Popover,
+    tags: ['autodocs'],
+    parameters: {
+        docs: {
+            description: {
+                component:
+                    'Anchored popover on the native Popover API (top-layer, light-dismiss, ESC, focus return). Hand-rolled side/align/offset positioning with flip + shift.',
+            },
+            story: { inline: false }, // Render stories in iframe for state isolation
+        },
+    },
+    argTypes: {
+        side: {
+            control: 'select',
+            options: ['top', 'bottom', 'left', 'right'],
+            description: 'Preferred side',
+        },
+        align: {
+            control: 'select',
+            options: ['start', 'center', 'end'],
+            description: 'Cross-axis alignment',
+        },
+        sideOffset: {
+            control: 'number',
+            description: 'Gap between trigger and content (px)',
+        },
+    },
+});
+</script>
+
+<script lang="ts">
+import { Slider } from '$shared/ui';
+
+let open = $state(false);
+let value = $state(50);
+</script>
+
+<Story name="Bottom">
+    {#snippet template()}
+        <div class="p-32 flex-center min-h-screen">
+            <Popover bind:open side="bottom" align="center" sideOffset={8}>
+                {#snippet trigger(props)}
+                    <button {...props} class="surface-card-elevated px-4 py-2">Open popover</button>
+                {/snippet}
+                {#snippet children()}
+                    <div class="surface-popover p-4 w-56">Popover content</div>
+                {/snippet}
+            </Popover>
+        </div>
+    {/snippet}
+</Story>
+
+<Story name="Top">
+    {#snippet template()}
+        <div class="p-32 flex-center min-h-screen">
+            <Popover bind:open side="top" align="center" sideOffset={8}>
+                {#snippet trigger(props)}
+                    <button {...props} class="surface-card-elevated px-4 py-2">Open popover</button>
+                {/snippet}
+                {#snippet children()}
+                    <div class="surface-popover p-4 w-56">Popover content</div>
+                {/snippet}
+            </Popover>
+        </div>
+    {/snippet}
+</Story>
+
+<!--
+    Mirrors TypographyMenu: top/end placement with a programmatic Close button
+    wired to the `close()` param of the children snippet.
+-->
+<Story name="AlignedEnd">
+    {#snippet template()}
+        <div class="p-32 flex-center min-h-screen">
+            <Popover bind:open side="top" align="end" sideOffset={8}>
+                {#snippet trigger(props)}
+                    <button {...props} class="surface-card-elevated px-4 py-2">Open menu</button>
+                {/snippet}
+                {#snippet children({ close })}
+                    <div class="surface-popover p-4 w-72">
+                        <h3 class="text-sm font-medium mb-3">Menu header</h3>
+                        <p class="text-sm text-muted-foreground mb-4">
+                            Aligned to the trigger's end edge.
+                        </p>
+                        <button class="surface-card-elevated px-3 py-1.5 text-sm" onclick={close}>
+                            Close
+                        </button>
+                    </div>
+                {/snippet}
+            </Popover>
+        </div>
+    {/snippet}
+</Story>
+
+<!-- Mirrors ComboControl: a vertical Slider lives inside the popover content. -->
+<Story name="WithSlider">
+    {#snippet template()}
+        <div class="p-32 flex-center min-h-screen">
+            <Popover bind:open side="top" align="center" sideOffset={8}>
+                {#snippet trigger(props)}
+                    <button {...props} class="surface-card-elevated px-4 py-2">Adjust value</button>
+                {/snippet}
+                {#snippet children()}
+                    <div class="surface-card-elevated p-3 h-64 flex-center">
+                        <Slider orientation="vertical" min={0} max={100} bind:value />
+                    </div>
+                {/snippet}
+            </Popover>
+        </div>
+    {/snippet}
+</Story>

src/shared/ui/Popover/Popover.svelte

@@ -0,0 +1,225 @@
+<!--
+    Component: Popover
+    Anchored popover on the native Popover API (top-layer, light-dismiss, ESC,
+    focus return handled by the browser). Placement is computed by the pure
+    `popover-position` module and applied as fixed coordinates; it repositions
+    on scroll/resize/content-resize. `open` is two-way bindable. The trigger is
+    consumer-rendered via the `trigger` snippet, which spreads a props object
+    (an attachment captures the trigger element; `popovertarget` wires the
+    native invoker). `children` receives `close()` to dismiss programmatically.
+-->
+<script lang="ts">
+import { cn } from '$shared/lib';
+import type { Snippet } from 'svelte';
+import { createAttachmentKey } from 'svelte/attachments';
+import {
+    type Align,
+    type Side,
+    computePosition,
+} from './popover-position';
+
+interface Props {
+    /**
+     * Open state (two-way bindable)
+     * @default false
+     */
+    open?: boolean;
+    /**
+     * Preferred side
+     * @default 'bottom'
+     */
+    side?: Side;
+    /**
+     * Cross-axis alignment
+     * @default 'center'
+     */
+    align?: Align;
+    /**
+     * Gap between trigger and content (px)
+     * @default 0
+     */
+    sideOffset?: number;
+    /**
+     * CSS classes applied to the content element
+     */
+    class?: string;
+    /**
+     * ARIA role for the content
+     * @default 'dialog'
+     */
+    role?: 'dialog' | 'menu' | 'listbox';
+    /**
+     * Trigger snippet — spread the provided props onto your trigger element
+     */
+    trigger: Snippet<[Record<string, unknown>]>;
+    /**
+     * Content snippet — receives `close()` for programmatic dismissal
+     */
+    children: Snippet<[{ close: () => void }]>;
+}
+
+let {
+    open = $bindable(false),
+    side = 'bottom',
+    align = 'center',
+    sideOffset = 0,
+    class: className,
+    role = 'dialog',
+    trigger,
+    children,
+}: Props = $props();
+
+const uid = $props.id();
+const contentId = `popover-${uid}`;
+
+let triggerEl: HTMLElement | undefined = $state();
+let contentEl: HTMLElement | undefined = $state();
+/**
+ * Side actually used after flip. Seeded from the `side` prop; the authoritative
+ * value is written by updatePosition() on every open, so the seed only matters
+ * for the closed state (hence the intentional state_referenced_locally warning).
+ */
+let resolvedSide = $state(side);
+
+/**
+ * True once updatePosition has applied coordinates for the current open.
+ * Gates visibility so the content never paints at its pre-positioned (0,0)
+ * top-layer default before the first measurement.
+ */
+let positioned = $state(false);
+
+/**
+ * Resolved fixed-position coordinates. Applied through the reactive `style`
+ * attribute (not imperatively) so they can't be wiped when the attribute
+ * re-renders — mixing the two caused a one-frame top-left flash.
+ */
+let x = $state(0);
+let y = $state(0);
+
+/**
+ * Actual DOM open state, driven by the `toggle` event. Source of truth for
+ * whether the browser currently shows the popover; `open` is the public binding.
+ */
+let shown = $state(false);
+
+/**
+ * Stable attachment that captures the consumer's trigger element for measuring.
+ * Created once so spreading reactive `triggerProps` doesn't re-run it.
+ */
+const attachKey = createAttachmentKey();
+const attachTrigger = (node: HTMLElement) => {
+    triggerEl = node;
+    return () => {
+        if (triggerEl === node) {
+            triggerEl = undefined;
+        }
+    };
+};
+
+const triggerProps = $derived({
+    popovertarget: contentId,
+    'aria-haspopup': role,
+    'aria-expanded': open,
+    'aria-controls': contentId,
+    [attachKey]: attachTrigger,
+});
+
+/**
+ * Recompute and apply the fixed-position coordinates.
+ */
+function updatePosition(): void {
+    if (!triggerEl || !contentEl) {
+        return;
+    }
+    const result = computePosition({
+        triggerRect: triggerEl.getBoundingClientRect(),
+        contentRect: { width: contentEl.offsetWidth, height: contentEl.offsetHeight },
+        viewport: { width: window.innerWidth, height: window.innerHeight },
+        side,
+        align,
+        sideOffset,
+    });
+    resolvedSide = result.side;
+    x = result.x;
+    y = result.y;
+    positioned = true;
+}
+
+/**
+ * Mirror the `toggle` event into our state.
+ */
+function onToggle(event: ToggleEvent): void {
+    shown = event.newState === 'open';
+    open = shown;
+    if (!shown) {
+        positioned = false;
+    }
+}
+
+/**
+ * Programmatic dismiss for the content snippet.
+ */
+function close(): void {
+    open = false;
+}
+
+// state -> browser: open the popover when `open` flips true and it isn't shown,
+// and close it when `open` flips false while shown. `shown` (from toggle) breaks
+// the loop so we never call show/hide redundantly.
+$effect(() => {
+    const el = contentEl;
+    if (!el) {
+        return;
+    }
+    if (open && !shown) {
+        el.showPopover();
+    } else if (!open && shown) {
+        el.hidePopover();
+    }
+});
+
+// Position while shown; reposition on scroll/resize/content-resize; auto-clean.
+$effect(() => {
+    if (!shown || !contentEl || !triggerEl) {
+        return;
+    }
+    updatePosition();
+    const observer = new ResizeObserver(() => updatePosition());
+    observer.observe(contentEl);
+    const onScroll = () => updatePosition();
+    window.addEventListener('scroll', onScroll, true);
+    window.addEventListener('resize', onScroll);
+    return () => {
+        observer.disconnect();
+        window.removeEventListener('scroll', onScroll, true);
+        window.removeEventListener('resize', onScroll);
+    };
+});
+</script>
+
+{@render trigger(triggerProps)}
+
+<!--
+    inset:auto + margin:0 neutralize the UA popover stylesheet (which sets
+    inset:0; margin:auto to center it) so the JS-applied left/top win.
+    visibility is hidden until updatePosition runs (see `positioned`).
+-->
+<div
+    bind:this={contentEl}
+    id={contentId}
+    popover="auto"
+    {role}
+    data-side={resolvedSide}
+    data-state={shown ? 'open' : 'closed'}
+    ontoggle={onToggle}
+    style={`position: fixed; inset: auto; left: ${x}px; top: ${y}px; margin: 0;${positioned ? '' : ' visibility: hidden;'}`}
+    class={cn(
+        'opacity-0 scale-95 transition-discrete transition-[opacity,transform] duration-fast',
+        'starting:opacity-0 starting:scale-95',
+        '[&:popover-open]:opacity-100 [&:popover-open]:scale-100',
+        'data-[side=top]:origin-bottom data-[side=bottom]:origin-top',
+        className,
+    )}
+>
+    {@render children({ close })}
+</div>

src/shared/ui/Popover/Popover.svelte.test.ts

@@ -0,0 +1,49 @@
+import {
+    fireEvent,
+    render,
+    screen,
+} from '@testing-library/svelte';
+import Harness from './PopoverHarness.svelte';
+
+/**
+ * Resolve the popover content element (the [popover] ancestor of the test content).
+ */
+function getContent(): HTMLElement {
+    return screen.getByTestId('content').closest('[popover]') as HTMLElement;
+}
+
+describe('Popover', () => {
+    it('renders the trigger with aria wiring, closed by default', () => {
+        render(Harness);
+        const trigger = screen.getByRole('button', { name: 'Open' });
+        expect(trigger).toHaveAttribute('aria-expanded', 'false');
+        expect(trigger).toHaveAttribute('aria-haspopup', 'dialog');
+        expect(trigger).toHaveAttribute('popovertarget');
+        expect(getContent()).toHaveAttribute('data-state', 'closed');
+    });
+
+    it('opens via the popover toggle and syncs aria-expanded + data-state', async () => {
+        render(Harness);
+        const trigger = screen.getByRole('button', { name: 'Open' });
+        // jsdom does not auto-invoke popovertarget; call the API the browser would.
+        getContent().showPopover();
+        await Promise.resolve();
+        expect(getContent()).toHaveAttribute('data-state', 'open');
+        expect(trigger).toHaveAttribute('aria-expanded', 'true');
+    });
+
+    it('opens when the parent sets open=true (state -> browser)', async () => {
+        render(Harness, { open: true });
+        await Promise.resolve();
+        expect(getContent()).toHaveAttribute('data-state', 'open');
+    });
+
+    it('close() hides the popover and resets aria-expanded', async () => {
+        render(Harness, { open: true });
+        await Promise.resolve();
+        const trigger = screen.getByRole('button', { name: 'Open' });
+        await fireEvent.click(screen.getByTestId('close'));
+        expect(getContent()).toHaveAttribute('data-state', 'closed');
+        expect(trigger).toHaveAttribute('aria-expanded', 'false');
+    });
+});

src/shared/ui/Popover/PopoverHarness.svelte

@@ -0,0 +1,21 @@
+<!--
+    Component: PopoverHarness
+    Test-only fixture: renders Popover with a button trigger and simple content
+    exposing the close() callback.
+-->
+<script lang="ts">
+import Popover from './Popover.svelte';
+
+let { open = $bindable(false) }: { open?: boolean } = $props();
+</script>
+
+<Popover bind:open>
+    {#snippet trigger(props)}
+        <button {...props}>Open</button>
+    {/snippet}
+    {#snippet children({ close })}
+        <div data-testid="content">
+            <button onclick={close} data-testid="close">Close</button>
+        </div>
+    {/snippet}
+</Popover>

src/shared/ui/Popover/popover-position.test.ts

@@ -0,0 +1,119 @@
+import {
+    type Align,
+    type Side,
+    computePosition,
+} from './popover-position';
+
+/**
+ * Build a DOMRect-like object (jsdom/node has no layout).
+ */
+function rect(x: number, y: number, width: number, height: number): DOMRect {
+    return {
+        x,
+        y,
+        width,
+        height,
+        top: y,
+        left: x,
+        right: x + width,
+        bottom: y + height,
+        toJSON: () => ({}),
+    } as DOMRect;
+}
+
+const viewport = { width: 1000, height: 800 };
+const content = { width: 200, height: 100 };
+
+function compute(side: Side, align: Align, sideOffset = 0, trigger = rect(400, 400, 100, 40)) {
+    return computePosition({ triggerRect: trigger, contentRect: content, viewport, side, align, sideOffset });
+}
+
+describe('computePosition', () => {
+    it('places below the trigger for side="bottom"', () => {
+        const r = compute('bottom', 'center');
+        expect(r.side).toBe('bottom');
+        expect(r.y).toBe(440); // trigger.bottom (400+40)
+    });
+
+    it('places above the trigger for side="top"', () => {
+        const r = compute('top', 'center');
+        expect(r.side).toBe('top');
+        expect(r.y).toBe(300); // trigger.top (400) - content.height (100)
+    });
+
+    it('applies sideOffset on the main axis', () => {
+        const r = compute('bottom', 'center', 8);
+        expect(r.y).toBe(448);
+    });
+
+    it('aligns center on the cross axis (vertical side)', () => {
+        const r = compute('bottom', 'center');
+        // trigger center x = 450; content half = 100 -> 350
+        expect(r.x).toBe(350);
+    });
+
+    it('aligns start and end on the cross axis (vertical side)', () => {
+        expect(compute('bottom', 'start').x).toBe(400); // trigger.left
+        expect(compute('bottom', 'end').x).toBe(300); // trigger.right(500) - content.width(200)
+    });
+
+    it('places left/right with vertical cross-axis alignment', () => {
+        const right = compute('right', 'start');
+        expect(right.side).toBe('right');
+        expect(right.x).toBe(500); // trigger.right
+        expect(right.y).toBe(400); // trigger.top (align start)
+        const left = compute('left', 'center');
+        expect(left.side).toBe('left');
+        expect(left.x).toBe(200); // trigger.left(400) - content.width(200)
+    });
+
+    it('flips top->bottom when there is no room above', () => {
+        const nearTop = rect(400, 20, 100, 40); // only 20px above, content needs 100
+        const r = computePosition({
+            triggerRect: nearTop,
+            contentRect: content,
+            viewport,
+            side: 'top',
+            align: 'center',
+            sideOffset: 0,
+        });
+        expect(r.side).toBe('bottom');
+        expect(r.y).toBe(60); // nearTop.bottom
+    });
+
+    it('does NOT flip when neither side fits (keeps requested side)', () => {
+        const tall = { width: 200, height: 700 };
+        const r = computePosition({
+            triggerRect: rect(400, 400, 100, 40),
+            contentRect: tall,
+            viewport,
+            side: 'top',
+            align: 'center',
+            sideOffset: 0,
+        });
+        expect(r.side).toBe('top');
+    });
+
+    it('shifts on the cross axis to stay within the viewport', () => {
+        const nearRight = rect(950, 400, 40, 40); // center x ~970, content 200 would overflow right
+        const r = computePosition({
+            triggerRect: nearRight,
+            contentRect: content,
+            viewport,
+            side: 'bottom',
+            align: 'center',
+            sideOffset: 0,
+        });
+        expect(r.x).toBe(800); // clamped to viewport.width(1000) - content.width(200)
+        const nearLeft = rect(10, 400, 40, 40);
+        const r2 = computePosition({
+            triggerRect: nearLeft,
+            contentRect: content,
+            viewport,
+            side: 'bottom',
+            align: 'center',
+            sideOffset: 0,
+        });
+        expect(r2.x).toBe(0); // clamped to 0
+    });
+});

src/shared/ui/Popover/popover-position.ts

@@ -0,0 +1,149 @@
+import { clampNumber } from '$shared/lib/utils';
+
+/**
+ * Side of the trigger the content prefers to open toward.
+ */
+export type Side = 'top' | 'bottom' | 'left' | 'right';
+
+/**
+ * Cross-axis alignment of the content relative to the trigger.
+ */
+export type Align = 'start' | 'center' | 'end';
+
+/**
+ * Inputs for a single placement computation. All geometry is injected
+ * (no DOM reads) so the function stays pure and unit-testable.
+ */
+type ComputeArgs = {
+    /**
+     * Trigger bounding rect (viewport coordinates).
+     */
+    triggerRect: DOMRect;
+    /**
+     * Measured content size.
+     */
+    contentRect: { width: number; height: number };
+    /**
+     * Viewport size.
+     */
+    viewport: { width: number; height: number };
+    /**
+     * Preferred side.
+     */
+    side: Side;
+    /**
+     * Cross-axis alignment.
+     */
+    align: Align;
+    /**
+     * Gap between trigger and content on the main axis.
+     */
+    sideOffset: number;
+};
+
+/**
+ * Resolved placement: fixed-position coordinates plus the side actually used
+ * (may differ from the requested side after a flip).
+ */
+type ComputeResult = { x: number; y: number; side: Side };
+
+const OPPOSITE: Record<Side, Side> = {
+    top: 'bottom',
+    bottom: 'top',
+    left: 'right',
+    right: 'left',
+};
+
+/**
+ * True for sides whose main axis is vertical (content sits above/below).
+ */
+function isVertical(side: Side): boolean {
+    return side === 'top' || side === 'bottom';
+}
+
+/**
+ * Main-axis coordinate (top for vertical sides, left for horizontal sides).
+ */
+function mainAxisCoord(side: Side, t: DOMRect, c: { width: number; height: number }, offset: number): number {
+    switch (side) {
+        case 'top':
+            return t.top - c.height - offset;
+        case 'bottom':
+            return t.bottom + offset;
+        case 'left':
+            return t.left - c.width - offset;
+        case 'right':
+            return t.right + offset;
+    }
+}
+
+/**
+ * Whether the content fits on the given side within the viewport.
+ */
+function fitsOnSide(
+    side: Side,
+    t: DOMRect,
+    c: { width: number; height: number },
+    v: { width: number; height: number },
+    offset: number,
+): boolean {
+    const coord = mainAxisCoord(side, t, c, offset);
+    switch (side) {
+        case 'top':
+            return coord >= 0;
+        case 'left':
+            return coord >= 0;
+        case 'bottom':
+            return coord + c.height <= v.height;
+        case 'right':
+            return coord + c.width <= v.width;
+    }
+}
+
+/**
+ * Cross-axis coordinate for the requested alignment.
+ */
+function crossAxisCoord(side: Side, align: Align, t: DOMRect, c: { width: number; height: number }): number {
+    if (isVertical(side)) {
+        if (align === 'start') {
+            return t.left;
+        }
+        if (align === 'end') {
+            return t.right - c.width;
+        }
+        return t.left + t.width / 2 - c.width / 2;
+    }
+    if (align === 'start') {
+        return t.top;
+    }
+    if (align === 'end') {
+        return t.bottom - c.height;
+    }
+    return t.top + t.height / 2 - c.height / 2;
+}
+
+/**
+ * Compute an anchored placement with flip (to the opposite side when the
+ * preferred side doesn't fit but the opposite does) and shift (clamp the
+ * cross axis so the content stays within the viewport).
+ */
+export function computePosition(args: ComputeArgs): ComputeResult {
+    const { triggerRect: t, contentRect: c, viewport: v, align, sideOffset } = args;
+    let side = args.side;
+
+    if (!fitsOnSide(side, t, c, v, sideOffset) && fitsOnSide(OPPOSITE[side], t, c, v, sideOffset)) {
+        side = OPPOSITE[side];
+    }
+
+    let x: number;
+    let y: number;
+    if (isVertical(side)) {
+        y = mainAxisCoord(side, t, c, sideOffset);
+        x = clampNumber(crossAxisCoord(side, align, t, c), 0, Math.max(0, v.width - c.width));
+    } else {
+        x = mainAxisCoord(side, t, c, sideOffset);
+        y = clampNumber(crossAxisCoord(side, align, t, c), 0, Math.max(0, v.height - c.height));
+    }
+
+    return { x, y, side };
+}