2026-04-08 06:34:20 +00:00
34 changed files with 2563 additions and 434 deletions
--- a/index.html
+++ b/index.html
@@ -4,6 +4,7 @@
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <title>glyphdiff</title>
+        <script src="https://mcp.figma.com/mcp/html-to-design/capture.js" async></script>
    </head>
    <body>
        <div id="app"></div>
--- a/src/entities/Font/index.ts
+++ b/src/entities/Font/index.ts
@@ -103,6 +103,11 @@ export {
    UNIFIED_FONTS,
 } from './lib/mocks';

+export {
+    FontNetworkError,
+    FontResponseError,
+} from './lib/errors/errors';
+
 // UI elements
 export {
    FontApplicator,
--- a/src/entities/Font/lib/errors/errors.test.ts
+++ b/src/entities/Font/lib/errors/errors.test.ts
@@ -0,0 +1,51 @@
+import {
+    FontNetworkError,
+    FontResponseError,
+} from './errors';
+
+describe('FontNetworkError', () => {
+    it('has correct name', () => {
+        const err = new FontNetworkError();
+        expect(err.name).toBe('FontNetworkError');
+    });
+
+    it('is instance of Error', () => {
+        expect(new FontNetworkError()).toBeInstanceOf(Error);
+    });
+
+    it('stores cause', () => {
+        const cause = new Error('network down');
+        const err = new FontNetworkError(cause);
+        expect(err.cause).toBe(cause);
+    });
+
+    it('has default message', () => {
+        expect(new FontNetworkError().message).toBe('Failed to fetch fonts from proxy API');
+    });
+});
+
+describe('FontResponseError', () => {
+    it('has correct name', () => {
+        const err = new FontResponseError('response', undefined);
+        expect(err.name).toBe('FontResponseError');
+    });
+
+    it('is instance of Error', () => {
+        expect(new FontResponseError('response.fonts', null)).toBeInstanceOf(Error);
+    });
+
+    it('stores field', () => {
+        const err = new FontResponseError('response.fonts', 42);
+        expect(err.field).toBe('response.fonts');
+    });
+
+    it('stores received value', () => {
+        const err = new FontResponseError('response.fonts', 42);
+        expect(err.received).toBe(42);
+    });
+
+    it('message includes field name', () => {
+        const err = new FontResponseError('response.fonts', null);
+        expect(err.message).toContain('response.fonts');
+    });
+});
--- a/src/entities/Font/lib/errors/errors.ts
+++ b/src/entities/Font/lib/errors/errors.ts
@@ -0,0 +1,28 @@
+/**
+ * Thrown when the network request to the proxy API fails.
+ * Wraps the underlying fetch error (timeout, DNS failure, connection refused, etc.).
+ */
+export class FontNetworkError extends Error {
+    readonly name = 'FontNetworkError';
+
+    constructor(public readonly cause?: unknown) {
+        super('Failed to fetch fonts from proxy API');
+    }
+}
+
+/**
+ * Thrown when the proxy API returns a response with an unexpected shape.
+ *
+ * @property field - The name of the field that failed validation (e.g. `'response'`, `'response.fonts'`).
+ * @property received - The actual value received at that field, for debugging.
+ */
+export class FontResponseError extends Error {
+    readonly name = 'FontResponseError';
+
+    constructor(
+        public readonly field: string,
+        public readonly received: unknown,
+    ) {
+        super(`Invalid proxy API response: ${field}`);
+    }
+}
--- a/src/entities/Font/lib/index.ts
+++ b/src/entities/Font/lib/index.ts
@@ -56,3 +56,8 @@ export {
    type MockUnifiedFontOptions,
    UNIFIED_FONTS,
 } from './mocks';
+
+export {
+    FontNetworkError,
+    FontResponseError,
+} from './errors/errors';
--- a/src/entities/Font/model/index.ts
+++ b/src/entities/Font/model/index.ts
@@ -8,6 +8,8 @@ export type {
    FontFeatures,
    FontFiles,
    FontItem,
+    FontLoadRequestConfig,
+    FontLoadStatus,
    FontMetadata,
    FontProvider,
    // Fontshare API types
@@ -37,7 +39,6 @@ export type {
 export {
    appliedFontsManager,
    createUnifiedFontStore,
-    type FontConfigRequest,
    type UnifiedFontStore,
    unifiedFontStore,
 } from './store';
--- a/src/entities/Font/model/store/appliedFontsStore/appliedFontStore.test.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/appliedFontStore.test.ts
@@ -1,73 +1,63 @@
 /** @vitest-environment jsdom */
-import {
-    afterEach,
-    beforeEach,
-    describe,
-    expect,
-    it,
-    vi,
-} from 'vitest';
 import { AppliedFontsManager } from './appliedFontsStore.svelte';
+import { FontFetchError } from './errors';
+import { FontEvictionPolicy } from './utils/fontEvictionPolicy/FontEvictionPolicy';
+
+// ── Fake collaborators ────────────────────────────────────────────────────────
+
+class FakeBufferCache {
+    async get(_url: string): Promise<ArrayBuffer> {
+        return new ArrayBuffer(8);
+    }
+    evict(_url: string): void {}
+    clear(): void {}
+}
+
+/** Throws {@link FontFetchError} on every `get()` — simulates network/HTTP failure. */
+class FailingBufferCache {
+    async get(url: string): Promise<never> {
+        throw new FontFetchError(url, new Error('network error'), 500);
+    }
+    evict(_url: string): void {}
+    clear(): void {}
+}
+
+// ── Helpers ───────────────────────────────────────────────────────────────────
+
+const makeConfig = (id: string, overrides: Partial<{ weight: number; isVariable: boolean }> = {}) => ({
+    id,
+    name: id,
+    url: `https://example.com/${id}.woff2`,
+    weight: 400,
+    ...overrides,
+});
+
+// ── Suite ─────────────────────────────────────────────────────────────────────

 describe('AppliedFontsManager', () => {
    let manager: AppliedFontsManager;
-    let mockFontFaceSet: any;
-    let mockFetch: any;
-    let failUrls: Set<string>;
+    let eviction: FontEvictionPolicy;
+    let mockFontFaceSet: { add: ReturnType<typeof vi.fn>; delete: ReturnType<typeof vi.fn> };

    beforeEach(() => {
        vi.useFakeTimers();
-        failUrls = new Set();
+        eviction = new FontEvictionPolicy({ ttl: 60000 });
+        mockFontFaceSet = { add: vi.fn(), delete: vi.fn() };

-        mockFontFaceSet = {
-            add: vi.fn(),
-            delete: vi.fn(),
-        };
-
-        // 1. Properly mock FontFace as a constructor function
-        // The actual implementation passes buffer (ArrayBuffer) as second arg, not URL string
-        const MockFontFace = vi.fn(function(this: any, name: string, bufferOrUrl: ArrayBuffer | string) {
-            this.name = name;
-            this.bufferOrUrl = bufferOrUrl;
-            this.load = vi.fn().mockImplementation(() => {
-                // For error tests, we track which URLs should fail via failUrls
-                // The fetch mock will have already rejected for those URLs
-                return Promise.resolve(this);
-            });
-        });
-
-        vi.stubGlobal('FontFace', MockFontFace);
-
-        // 2. Mock document.fonts safely
        Object.defineProperty(document, 'fonts', {
            value: mockFontFaceSet,
            configurable: true,
            writable: true,
        });

-        vi.stubGlobal('crypto', {
-            randomUUID: () => '11111111-1111-1111-1111-111111111111' as any,
+        const MockFontFace = vi.fn(function(this: any, name: string, buffer: BufferSource) {
+            this.name = name;
+            this.buffer = buffer;
+            this.load = vi.fn().mockResolvedValue(this);
        });
+        vi.stubGlobal('FontFace', MockFontFace);

-        // 3. Mock fetch to return fake ArrayBuffer data
-        mockFetch = vi.fn((url: string) => {
-            if (failUrls.has(url)) {
-                return Promise.reject(new Error('Network error'));
-            }
-            return Promise.resolve({
-                ok: true,
-                status: 200,
-                arrayBuffer: () => Promise.resolve(new ArrayBuffer(8)),
-                clone: () => ({
-                    ok: true,
-                    status: 200,
-                    arrayBuffer: () => Promise.resolve(new ArrayBuffer(8)),
-                }),
-            } as Response);
-        });
-        vi.stubGlobal('fetch', mockFetch);
-
-        manager = new AppliedFontsManager();
+        manager = new AppliedFontsManager({ cache: new FakeBufferCache() as any, eviction });
    });

    afterEach(() => {
@@ -76,138 +66,267 @@ describe('AppliedFontsManager', () => {
        vi.unstubAllGlobals();
    });

-    it('should batch multiple font requests into a single process', async () => {
-        const configs = [
-            { id: 'lato-400', name: 'Lato', url: 'https://example.com/lato.ttf', weight: 400 },
-            { id: 'lato-700', name: 'Lato', url: 'https://example.com/lato-bold.ttf', weight: 700 },
-        ];
+    // ── touch() ───────────────────────────────────────────────────────────────

-        manager.touch(configs);
+    describe('touch()', () => {
+        it('queues and loads a new font', async () => {
+            manager.touch([makeConfig('roboto')]);
+            await vi.advanceTimersByTimeAsync(50);

-        // Advance to trigger the 16ms debounced #processQueue
-        await vi.advanceTimersByTimeAsync(50);
+            expect(manager.getFontStatus('roboto', 400)).toBe('loaded');
+        });

-        expect(manager.getFontStatus('lato-400', 400)).toBe('loaded');
-        expect(mockFontFaceSet.add).toHaveBeenCalledTimes(2);
+        it('batches multiple fonts into a single queue flush', async () => {
+            manager.touch([makeConfig('lato'), makeConfig('inter')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(mockFontFaceSet.add).toHaveBeenCalledTimes(2);
+        });
+
+        it('skips fonts that are already loaded', async () => {
+            manager.touch([makeConfig('lato')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            manager.touch([makeConfig('lato')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(mockFontFaceSet.add).toHaveBeenCalledTimes(1);
+        });
+
+        it('skips fonts that are currently loading', async () => {
+            manager.touch([makeConfig('lato')]);
+            // simulate loading state before queue drains
+            manager.statuses.set('lato@400', 'loading');
+            manager.touch([makeConfig('lato')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(mockFontFaceSet.add).toHaveBeenCalledTimes(1);
+        });
+
+        it('skips fonts that have exhausted retries', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const failManager = new AppliedFontsManager({ cache: new FailingBufferCache() as any, eviction });
+
+            // exhaust all 3 retries
+            for (let i = 0; i < 3; i++) {
+                failManager.statuses.delete('broken@400');
+                failManager.touch([makeConfig('broken')]);
+                await vi.advanceTimersByTimeAsync(50);
+            }
+
+            failManager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(failManager.getFontStatus('broken', 400)).toBe('error');
+            expect(mockFontFaceSet.add).not.toHaveBeenCalled();
+            consoleSpy.mockRestore();
+        });
+
+        it('does nothing after manager is destroyed', async () => {
+            manager.destroy();
+            manager.touch([makeConfig('roboto')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(manager.statuses.size).toBe(0);
+        });
    });

-    it('should handle font loading errors gracefully', async () => {
-        // Suppress expected console error for clean test logs
-        const spy = vi.spyOn(console, 'error').mockImplementation(() => {});
+    // ── queue processing ──────────────────────────────────────────────────────

-        const failUrl = 'https://example.com/fail.ttf';
-        failUrls.add(failUrl);
+    describe('queue processing', () => {
+        it('filters non-critical weights in data-saver mode', async () => {
+            (navigator as any).connection = { saveData: true };

-        const config = { id: 'broken', name: 'Broken', url: failUrl, weight: 400 };
+            manager.touch([
+                makeConfig('light', { weight: 300 }),
+                makeConfig('regular', { weight: 400 }),
+                makeConfig('bold', { weight: 700 }),
+            ]);
+            await vi.advanceTimersByTimeAsync(50);

-        manager.touch([config]);
-        await vi.advanceTimersByTimeAsync(50);
+            expect(manager.getFontStatus('light', 300)).toBeUndefined();
+            expect(manager.getFontStatus('regular', 400)).toBe('loaded');
+            expect(manager.getFontStatus('bold', 700)).toBe('loaded');

-        expect(manager.getFontStatus('broken', 400)).toBe('error');
-        spy.mockRestore();
+            delete (navigator as any).connection;
+        });
+
+        it('loads variable fonts in data-saver mode regardless of weight', async () => {
+            (navigator as any).connection = { saveData: true };
+
+            manager.touch([makeConfig('vf', { weight: 300, isVariable: true })]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(manager.getFontStatus('vf', 300, true)).toBe('loaded');
+
+            delete (navigator as any).connection;
+        });
    });

-    it('should purge fonts after TTL expires', async () => {
-        const config = { id: 'ephemeral', name: 'Temp', url: 'https://example.com/temp.ttf', weight: 400 };
+    // ── Phase 1: fetch ────────────────────────────────────────────────────────

-        manager.touch([config]);
-        await vi.advanceTimersByTimeAsync(50);
-        expect(manager.getFontStatus('ephemeral', 400)).toBe('loaded');
+    describe('Phase 1 — fetch', () => {
+        it('sets status to error on fetch failure', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const failManager = new AppliedFontsManager({ cache: new FailingBufferCache() as any, eviction });

-        // Move clock forward past TTL (5m) and Purge Interval (1m)
-        // advanceTimersByTimeAsync is key here; it handles the promises inside the interval
-        await vi.advanceTimersByTimeAsync(6 * 60 * 1000);
+            failManager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);

-        expect(manager.getFontStatus('ephemeral', 400)).toBeUndefined();
-        expect(mockFontFaceSet.delete).toHaveBeenCalled();
+            expect(failManager.getFontStatus('broken', 400)).toBe('error');
+            consoleSpy.mockRestore();
+        });
+
+        it('logs a console error on fetch failure', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const failManager = new AppliedFontsManager({ cache: new FailingBufferCache() as any, eviction });
+
+            failManager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(consoleSpy).toHaveBeenCalled();
+            consoleSpy.mockRestore();
+        });
+
+        it('does not set error status or log for aborted fetches', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const abortingCache = {
+                async get(url: string): Promise<never> {
+                    throw new FontFetchError(url, Object.assign(new Error('Aborted'), { name: 'AbortError' }));
+                },
+                evict() {},
+                clear() {},
+            };
+            const abortManager = new AppliedFontsManager({ cache: abortingCache as any, eviction });
+
+            abortManager.touch([makeConfig('aborted')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            // status is left as 'loading' (not 'error') — abort is not a retriable failure
+            expect(abortManager.getFontStatus('aborted', 400)).not.toBe('error');
+            expect(consoleSpy).not.toHaveBeenCalled();
+            consoleSpy.mockRestore();
+        });
    });

-    it('should NOT purge fonts that are still being "touched"', async () => {
-        const config = { id: 'active', name: 'Active', url: 'https://example.com/active.ttf', weight: 400 };
+    // ── Phase 2: parse ────────────────────────────────────────────────────────

-        manager.touch([config]);
-        await vi.advanceTimersByTimeAsync(50);
+    describe('Phase 2 — parse', () => {
+        it('sets status to error on parse failure', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const FailingFontFace = vi.fn(function(this: any) {
+                this.load = vi.fn().mockRejectedValue(new Error('parse failed'));
+            });
+            vi.stubGlobal('FontFace', FailingFontFace);

-        // Advance 4 minutes
-        await vi.advanceTimersByTimeAsync(4 * 60 * 1000);
+            manager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);

-        // Refresh touch
-        manager.touch([config]);
+            expect(manager.getFontStatus('broken', 400)).toBe('error');
+            consoleSpy.mockRestore();
+        });

-        // Advance another 2 minutes (Total 6 since start)
-        await vi.advanceTimersByTimeAsync(2 * 60 * 1000);
+        it('logs a console error on parse failure', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const FailingFontFace = vi.fn(function(this: any) {
+                this.load = vi.fn().mockRejectedValue(new Error('parse failed'));
+            });
+            vi.stubGlobal('FontFace', FailingFontFace);

-        expect(manager.getFontStatus('active', 400)).toBe('loaded');
+            manager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(consoleSpy).toHaveBeenCalled();
+            consoleSpy.mockRestore();
+        });
    });

-    it('should serve buffer from memory without calling fetch again', async () => {
-        const config = { id: 'cached', name: 'Cached', url: 'https://example.com/cached.ttf', weight: 400 };
+    // ── #purgeUnused ──────────────────────────────────────────────────────────

-        // First load — populates in-memory buffer
-        manager.touch([config]);
-        await vi.advanceTimersByTimeAsync(50);
-        expect(manager.getFontStatus('cached', 400)).toBe('loaded');
-        expect(mockFetch).toHaveBeenCalledTimes(1);
+    describe('#purgeUnused', () => {
+        it('evicts fonts after TTL expires', async () => {
+            manager.touch([makeConfig('ephemeral')]);
+            await vi.advanceTimersByTimeAsync(50);

-        // Simulate eviction by deleting the status entry directly
-        manager.statuses.delete('cached@400');
+            await vi.advanceTimersByTimeAsync(61000);

-        // Second load — should hit in-memory buffer, not network
-        manager.touch([config]);
-        await vi.advanceTimersByTimeAsync(50);
+            expect(manager.getFontStatus('ephemeral', 400)).toBeUndefined();
+            expect(mockFontFaceSet.delete).toHaveBeenCalled();
+        });

-        expect(manager.getFontStatus('cached', 400)).toBe('loaded');
-        // fetch should still only have been called once (buffer was reused)
-        expect(mockFetch).toHaveBeenCalledTimes(1);
+        it('removes the evicted key from the eviction policy', async () => {
+            manager.touch([makeConfig('ephemeral')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            await vi.advanceTimersByTimeAsync(61000);
+
+            expect(Array.from(eviction.keys())).not.toContain('ephemeral@400');
+        });
+
+        it('refreshes TTL when font is re-touched before expiry', async () => {
+            const config = makeConfig('active');
+            manager.touch([config]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            await vi.advanceTimersByTimeAsync(40000);
+            manager.touch([config]); // refresh at t≈40s
+
+            await vi.advanceTimersByTimeAsync(25000); // purge at t≈60s sees only ~20s elapsed → not evicted
+
+            expect(manager.getFontStatus('active', 400)).toBe('loaded');
+        });
+
+        it('does not evict pinned fonts', async () => {
+            manager.touch([makeConfig('pinned')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            manager.pin('pinned', 400);
+            await vi.advanceTimersByTimeAsync(61000);
+
+            expect(manager.getFontStatus('pinned', 400)).toBe('loaded');
+            expect(mockFontFaceSet.delete).not.toHaveBeenCalled();
+        });
+
+        it('evicts font after it is unpinned and TTL expires', async () => {
+            manager.touch([makeConfig('toggled')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            manager.pin('toggled', 400);
+            manager.unpin('toggled', 400);
+            await vi.advanceTimersByTimeAsync(61000);
+
+            expect(manager.getFontStatus('toggled', 400)).toBeUndefined();
+            expect(mockFontFaceSet.delete).toHaveBeenCalled();
+        });
    });

-    it('should NOT purge a pinned font after TTL expires', async () => {
-        const config = { id: 'pinned', name: 'Pinned', url: 'https://example.com/pinned.ttf', weight: 400 };
+    // ── destroy() ─────────────────────────────────────────────────────────────

-        manager.touch([config]);
-        await vi.advanceTimersByTimeAsync(50);
-        expect(manager.getFontStatus('pinned', 400)).toBe('loaded');
+    describe('destroy()', () => {
+        it('clears all statuses', async () => {
+            manager.touch([makeConfig('roboto')]);
+            await vi.advanceTimersByTimeAsync(50);

-        manager.pin('pinned', 400);
+            manager.destroy();

-        // Advance past TTL + purge interval
-        await vi.advanceTimersByTimeAsync(6 * 60 * 1000);
+            expect(manager.statuses.size).toBe(0);
+        });

-        expect(manager.getFontStatus('pinned', 400)).toBe('loaded');
-        expect(mockFontFaceSet.delete).not.toHaveBeenCalled();
-    });
+        it('removes all loaded fonts from document.fonts', async () => {
+            manager.touch([makeConfig('roboto'), makeConfig('inter')]);
+            await vi.advanceTimersByTimeAsync(50);

-    it('should evict a font after it is unpinned and TTL expires', async () => {
-        const config = { id: 'unpinned', name: 'Unpinned', url: 'https://example.com/unpinned.ttf', weight: 400 };
+            manager.destroy();

-        manager.touch([config]);
-        await vi.advanceTimersByTimeAsync(50);
-        expect(manager.getFontStatus('unpinned', 400)).toBe('loaded');
+            expect(mockFontFaceSet.delete).toHaveBeenCalledTimes(2);
+        });

-        manager.pin('unpinned', 400);
-        manager.unpin('unpinned', 400);
+        it('prevents further loading after destroy', async () => {
+            manager.destroy();
+            manager.touch([makeConfig('roboto')]);
+            await vi.advanceTimersByTimeAsync(50);

-        // Advance past TTL + purge interval
-        await vi.advanceTimersByTimeAsync(6 * 60 * 1000);
-
-        expect(manager.getFontStatus('unpinned', 400)).toBeUndefined();
-        expect(mockFontFaceSet.delete).toHaveBeenCalled();
-    });
-
-    it('should clear pinned set on destroy without errors', async () => {
-        const config = {
-            id: 'destroy-pin',
-            name: 'DestroyPin',
-            url: 'https://example.com/destroypin.ttf',
-            weight: 400,
-        };
-
-        manager.touch([config]);
-        await vi.advanceTimersByTimeAsync(50);
-
-        manager.pin('destroy-pin', 400);
-        manager.destroy();
-
-        expect(manager.statuses.size).toBe(0);
+            expect(manager.statuses.size).toBe(0);
+        });
    });
 });
--- a/src/entities/Font/model/store/appliedFontsStore/appliedFontsStore.svelte.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/appliedFontsStore.svelte.ts
@@ -1,30 +1,26 @@
 import { SvelteMap } from 'svelte/reactivity';
+import {
+    type FontLoadRequestConfig,
+    type FontLoadStatus,
+} from '../../types';
+import {
+    FontFetchError,
+    FontParseError,
+} from './errors';
+import {
+    generateFontKey,
+    getEffectiveConcurrency,
+    loadFont,
+    yieldToMainThread,
+} from './utils';
+import { FontBufferCache } from './utils/fontBufferCache/FontBufferCache';
+import { FontEvictionPolicy } from './utils/fontEvictionPolicy/FontEvictionPolicy';
+import { FontLoadQueue } from './utils/fontLoadQueue/FontLoadQueue';

-/** Loading state of a font. Failed loads may be retried up to MAX_RETRIES. */
-export type FontStatus = 'loading' | 'loaded' | 'error';
-
-/** Configuration for a font load request. */
-export interface FontConfigRequest {
-    /**
-     * Unique identifier for the font (e.g., "lato", "roboto").
-     */
-    id: string;
-    /**
-     * Actual font family name recognized by the browser (e.g., "Lato", "Roboto").
-     */
-    name: string;
-    /**
-     * URL pointing to the font file (typically .ttf or .woff2).
-     */
-    url: string;
-    /**
-     * Numeric weight (100-900). Variable fonts load once per ID regardless of weight.
-     */
-    weight: number;
-    /**
-     * Variable fonts load once per ID; static fonts load per weight.
-     */
-    isVariable?: boolean;
+interface AppliedFontsManagerDeps {
+    cache?: FontBufferCache;
+    eviction?: FontEvictionPolicy;
+    queue?: FontLoadQueue;
 }

 /**
@@ -51,14 +47,16 @@ export interface FontConfigRequest {
 * **Browser APIs Used:** `scheduler.yield()`, `isInputPending()`, `requestIdleCallback`, Cache API, Network Information API
 */
 export class AppliedFontsManager {
+    // Injected collaborators - each handles one concern for better testability
+    readonly #cache: FontBufferCache;
+    readonly #eviction: FontEvictionPolicy;
+    readonly #queue: FontLoadQueue;
+
    // Loaded FontFace instances registered with document.fonts. Key: `{id}@{weight}` or `{id}@vf`
    #loadedFonts = new Map<string, FontFace>();

-    // Last-used timestamps for LRU cleanup. Key: `{id}@{weight}` or `{id}@vf`, Value: unix timestamp (ms)
-    #usageTracker = new Map<string, number>();
-
-    // Fonts queued for loading by `touch()`, processed by `#processQueue()`
-    #queue = new Map<string, FontConfigRequest>();
+    // Maps font key → URL so #purgeUnused() can evict from cache
+    #urlByKey = new Map<string, string>();

    // Handle for scheduled queue processing (requestIdleCallback or setTimeout)
    #timeoutId: ReturnType<typeof setTimeout> | null = null;
@@ -72,112 +70,95 @@ export class AppliedFontsManager {
    // Tracks which callback type is pending ('idle' | 'timeout' | null) for proper cancellation
    #pendingType: 'idle' | 'timeout' | null = null;

-    // Retry counts for failed loads; fonts exceeding MAX_RETRIES are permanently skipped
-    #retryCounts = new Map<string, number>();
-
-    // In-memory buffer cache keyed by URL — fastest tier, checked before Cache API and network
-    #buffersByUrl = new Map<string, ArrayBuffer>();
-
-    // Maps font key → URL so #purgeUnused() can evict from #buffersByUrl
-    #urlByKey = new Map<string, string>();
-
-    // Fonts currently visible/in-use; purge skips these regardless of TTL
-    #pinnedFonts = new Set<string>();
-
-    readonly #MAX_RETRIES = 3;
-    readonly #PURGE_INTERVAL = 60000; // 60 seconds
-    readonly #TTL = 5 * 60 * 1000; // 5 minutes
-    readonly #CACHE_NAME = 'font-cache-v1'; // Versioned for future invalidation
+    readonly #PURGE_INTERVAL = 60000;

    // Reactive status map for Svelte components to track font states
-    statuses = new SvelteMap<string, FontStatus>();
+    statuses = new SvelteMap<string, FontLoadStatus>();

    // Starts periodic cleanup timer (browser-only).
-    constructor() {
+    constructor(
+        { cache = new FontBufferCache(), eviction = new FontEvictionPolicy(), queue = new FontLoadQueue() }:
+            AppliedFontsManagerDeps = {},
+    ) {
+        // Inject collaborators - defaults provided for production, fakes for testing
+        this.#cache = cache;
+        this.#eviction = eviction;
+        this.#queue = queue;
        if (typeof window !== 'undefined') {
            this.#intervalId = setInterval(() => this.#purgeUnused(), this.#PURGE_INTERVAL);
        }
    }

-    // Generates font key: `{id}@vf` for variable, `{id}@{weight}` for static.
-    #getFontKey(id: string, weight: number, isVariable: boolean): string {
-        return isVariable ? `${id.toLowerCase()}@vf` : `${id.toLowerCase()}@${weight}`;
-    }
-
    /**
     * Requests fonts to be loaded. Updates usage tracking and queues new fonts.
     *
     * Retry behavior: 'loaded' and 'loading' fonts are skipped; 'error' fonts retry if count < MAX_RETRIES.
     * Scheduling: Prefers requestIdleCallback (150ms timeout), falls back to setTimeout(16ms).
     */
-    touch(configs: FontConfigRequest[]) {
-        if (this.#abortController.signal.aborted) return;
-
-        const now = Date.now();
-        let hasNewItems = false;
-
-        for (const config of configs) {
-            const key = this.#getFontKey(config.id, config.weight, !!config.isVariable);
-            this.#usageTracker.set(key, now);
-
-            const status = this.statuses.get(key);
-            if (status === 'loaded' || status === 'loading' || this.#queue.has(key)) continue;
-            if (status === 'error' && (this.#retryCounts.get(key) ?? 0) >= this.#MAX_RETRIES) continue;
-
-            this.#queue.set(key, config);
-            hasNewItems = true;
+    touch(configs: FontLoadRequestConfig[]) {
+        if (this.#abortController.signal.aborted) {
+            return;
        }
+        try {
+            const now = Date.now();
+            let hasNewItems = false;

-        if (hasNewItems && !this.#timeoutId) {
-            if (typeof requestIdleCallback !== 'undefined') {
-                this.#timeoutId = requestIdleCallback(
-                    () => this.#processQueue(),
-                    { timeout: 150 },
-                ) as unknown as ReturnType<typeof setTimeout>;
-                this.#pendingType = 'idle';
-            } else {
-                this.#timeoutId = setTimeout(() => this.#processQueue(), 16);
-                this.#pendingType = 'timeout';
+            for (const config of configs) {
+                const key = generateFontKey(config);
+
+                // Update last-used timestamp for LRU eviction policy
+                this.#eviction.touch(key, now);
+
+                const status = this.statuses.get(key);
+
+                // Skip fonts that are already loaded or currently loading
+                if (status === 'loaded' || status === 'loading') {
+                    continue;
+                }
+
+                // Skip fonts already in the queue (avoid duplicates)
+                if (this.#queue.has(key)) {
+                    continue;
+                }
+
+                // Skip error fonts that have exceeded max retry count
+                if (status === 'error' && this.#queue.isMaxRetriesReached(key)) {
+                    continue;
+                }
+
+                // Queue this font for loading
+                this.#queue.enqueue(key, config);
+                hasNewItems = true;
            }
+
+            if (hasNewItems && !this.#timeoutId) {
+                this.#scheduleProcessing();
+            }
+        } catch (error) {
+            console.error(error);
        }
    }

-    /** Yields to main thread during CPU-intensive parsing. Uses scheduler.yield() (Chrome/Edge) or MessageChannel fallback. */
-    async #yieldToMain(): Promise<void> {
-        // @ts-expect-error - scheduler not in TypeScript lib yet
-        if (typeof scheduler !== 'undefined' && 'yield' in scheduler) {
-            // @ts-expect-error - scheduler.yield not in TypeScript lib yet
-            await scheduler.yield();
+    /**
+     * Schedules `#processQueue()` via `requestIdleCallback` (150ms timeout) when available,
+     * falling back to `setTimeout(16ms)` for ~60fps timing.
+     */
+    #scheduleProcessing(): void {
+        if (typeof requestIdleCallback !== 'undefined') {
+            this.#timeoutId = requestIdleCallback(
+                () => this.#processQueue(),
+                { timeout: 150 },
+            ) as unknown as ReturnType<typeof setTimeout>;
+            this.#pendingType = 'idle';
        } else {
-            await new Promise<void>(resolve => {
-                const ch = new MessageChannel();
-                ch.port1.onmessage = () => resolve();
-                ch.port2.postMessage(null);
-            });
-        }
-    }
-
-    /** Returns optimal concurrent fetches based on Network Information API: 1 for 2G, 2 for 3G, 4 for 4G/default. */
-    #getEffectiveConcurrency(): number {
-        const nav = navigator as any;
-        const conn = nav.connection;
-        if (!conn) return 4;
-
-        switch (conn.effectiveType) {
-            case 'slow-2g':
-            case '2g':
-                return 1;
-            case '3g':
-                return 2;
-            default:
-                return 4;
+            this.#timeoutId = setTimeout(() => this.#processQueue(), 16);
+            this.#pendingType = 'timeout';
        }
    }

    /** Returns true if data-saver mode is enabled (defers non-critical weights). */
    #shouldDeferNonCritical(): boolean {
-        const nav = navigator as any;
-        return nav.connection?.saveData === true;
+        return (navigator as any).connection?.saveData === true;
    }

    /**
@@ -188,173 +169,179 @@ export class AppliedFontsManager {
     * Yielding: Chromium uses `isInputPending()` for optimal responsiveness; others yield every 8ms.
     */
    async #processQueue() {
+        // Clear timer flags since we're now processing
        this.#timeoutId = null;
        this.#pendingType = null;

-        let entries = Array.from(this.#queue.entries());
-        if (!entries.length) return;
-        this.#queue.clear();
+        // Get all queued entries and clear the queue atomically
+        let entries = this.#queue.flush();
+        if (!entries.length) {
+            return;
+        }

+        // In data-saver mode, only load variable fonts and common weights (400, 700)
        if (this.#shouldDeferNonCritical()) {
            entries = entries.filter(([, c]) => c.isVariable || [400, 700].includes(c.weight));
        }

-        // Phase 1: Concurrent fetching (I/O bound, non-blocking)
-        const concurrency = this.#getEffectiveConcurrency();
+        // Determine optimal concurrent fetches based on network speed (1-4)
+        const concurrency = getEffectiveConcurrency();
        const buffers = new Map<string, ArrayBuffer>();

+        // ==================== PHASE 1: Concurrent Fetching ====================
+        // Fetch multiple font files in parallel since network I/O is non-blocking
        for (let i = 0; i < entries.length; i += concurrency) {
-            const chunk = entries.slice(i, i + concurrency);
-            const results = await Promise.allSettled(
-                chunk.map(async ([key, config]) => {
-                    this.statuses.set(key, 'loading');
-                    const buffer = await this.#fetchFontBuffer(
-                        config.url,
-                        this.#abortController.signal,
-                    );
-                    buffers.set(key, buffer);
-                }),
-            );
-
-            for (let j = 0; j < results.length; j++) {
-                if (results[j].status === 'rejected') {
-                    const [key, config] = chunk[j];
-                    console.error(`Font fetch failed: ${config.name}`, (results[j] as PromiseRejectedResult).reason);
-                    this.statuses.set(key, 'error');
-                    this.#retryCounts.set(key, (this.#retryCounts.get(key) ?? 0) + 1);
-                }
-            }
+            await this.#fetchChunk(entries.slice(i, i + concurrency), buffers);
        }

-        // Phase 2: Sequential parsing (CPU-intensive, yields periodically)
+        // ==================== PHASE 2: Sequential Parsing ====================
+        // Parse buffers one at a time with periodic yields to avoid blocking UI
        const hasInputPending = !!(navigator as any).scheduling?.isInputPending;
        let lastYield = performance.now();
-        const YIELD_INTERVAL = 8; // ms
+        const YIELD_INTERVAL = 8;

        for (const [key, config] of entries) {
            const buffer = buffers.get(key);
-            if (!buffer) continue;
-
-            try {
-                const weightRange = config.isVariable ? '100 900' : `${config.weight}`;
-                const font = new FontFace(config.name, buffer, {
-                    weight: weightRange,
-                    style: 'normal',
-                    display: 'swap',
-                });
-                await font.load();
-                document.fonts.add(font);
-                this.#loadedFonts.set(key, font);
-                this.#buffersByUrl.set(config.url, buffer);
-                this.#urlByKey.set(key, config.url);
-                this.statuses.set(key, 'loaded');
-            } catch (e) {
-                if (e instanceof Error && e.name === 'AbortError') continue;
-                console.error(`Font parse failed: ${config.name}`, e);
-                this.statuses.set(key, 'error');
-                this.#retryCounts.set(key, (this.#retryCounts.get(key) ?? 0) + 1);
+            // Skip fonts that failed to fetch in phase 1
+            if (!buffer) {
+                continue;
            }

+            await this.#processFont(key, config, buffer);
+
+            // Yield to main thread if needed (prevents UI blocking)
+            // Chromium: use isInputPending() for optimal responsiveness
+            // Others: yield every 8ms as fallback
            const shouldYield = hasInputPending
                ? (navigator as any).scheduling.isInputPending({ includeContinuous: true })
-                : (performance.now() - lastYield > YIELD_INTERVAL);
+                : performance.now() - lastYield > YIELD_INTERVAL;

            if (shouldYield) {
-                await this.#yieldToMain();
+                await yieldToMainThread();
                lastYield = performance.now();
            }
        }
    }

    /**
-     * Fetches font with three-tier lookup: in-memory buffer → Cache API → network.
-     * Cache failures (private browsing, quota limits) are silently ignored.
+     * Fetches a chunk of fonts concurrently and populates `buffers` with successful results.
+     * Each promise carries its own key and config so results need no index correlation.
+     * Aborted fetches are silently skipped; other errors set status to `'error'` and increment retry.
     */
-    async #fetchFontBuffer(url: string, signal?: AbortSignal): Promise<ArrayBuffer> {
-        // Tier 1: in-memory buffer (fastest, no I/O)
-        const inMemory = this.#buffersByUrl.get(url);
-        if (inMemory) return inMemory;
+    async #fetchChunk(
+        chunk: Array<[string, FontLoadRequestConfig]>,
+        buffers: Map<string, ArrayBuffer>,
+    ): Promise<void> {
+        const results = await Promise.all(
+            chunk.map(async ([key, config]) => {
+                this.statuses.set(key, 'loading');
+                try {
+                    const buffer = await this.#cache.get(config.url, this.#abortController.signal);
+                    buffers.set(key, buffer);
+                    return { ok: true as const, key };
+                } catch (reason) {
+                    return { ok: false as const, key, config, reason };
+                }
+            }),
+        );

-        // Tier 2: Cache API
-        try {
-            if (typeof caches !== 'undefined') {
-                const cache = await caches.open(this.#CACHE_NAME);
-                const cached = await cache.match(url);
-                if (cached) return cached.arrayBuffer();
+        for (const result of results) {
+            if (result.ok) continue;
+            const { key, config, reason } = result;
+            const isAbort = reason instanceof FontFetchError
+                && reason.cause instanceof Error
+                && reason.cause.name === 'AbortError';
+            if (isAbort) continue;
+            if (reason instanceof FontFetchError) {
+                console.error(`Font fetch failed: ${config.name}`, reason);
            }
-        } catch {
-            // Cache unavailable (private browsing, security restrictions) — fall through to network
+            this.statuses.set(key, 'error');
+            this.#queue.incrementRetry(key);
        }
+    }

-        // Tier 3: network
-        const response = await fetch(url, { signal });
-        if (!response.ok) throw new Error(`HTTP ${response.status}`);
-
+    /**
+     * Parses a fetched buffer into a {@link FontFace}, registers it with `document.fonts`,
+     * and updates reactive status. On failure, sets status to `'error'` and increments the retry count.
+     */
+    async #processFont(key: string, config: FontLoadRequestConfig, buffer: ArrayBuffer): Promise<void> {
        try {
-            if (typeof caches !== 'undefined') {
-                const cache = await caches.open(this.#CACHE_NAME);
-                await cache.put(url, response.clone());
+            const font = await loadFont(config, buffer);
+            this.#loadedFonts.set(key, font);
+            this.#urlByKey.set(key, config.url);
+            this.statuses.set(key, 'loaded');
+        } catch (e) {
+            if (e instanceof FontParseError) {
+                console.error(`Font parse failed: ${config.name}`, e);
+                this.statuses.set(key, 'error');
+                this.#queue.incrementRetry(key);
            }
-        } catch {
-            // Cache write failed (quota, storage pressure) — return font anyway
        }
-
-        return response.arrayBuffer();
    }

    /** Removes fonts unused within TTL (LRU-style cleanup). Runs every PURGE_INTERVAL. Pinned fonts are never evicted. */
    #purgeUnused() {
        const now = Date.now();
-        for (const [key, lastUsed] of this.#usageTracker) {
-            if (now - lastUsed < this.#TTL) continue;
-            if (this.#pinnedFonts.has(key)) continue;
+        // Iterate through all tracked font keys
+        for (const key of this.#eviction.keys()) {
+            // Skip fonts that are still within TTL or are pinned
+            if (!this.#eviction.shouldEvict(key, now)) {
+                continue;
+            }

+            // Remove FontFace from document to free memory
            const font = this.#loadedFonts.get(key);
            if (font) document.fonts.delete(font);

+            // Evict from cache and cleanup URL mapping
            const url = this.#urlByKey.get(key);
            if (url) {
-                this.#buffersByUrl.delete(url);
+                this.#cache.evict(url);
                this.#urlByKey.delete(key);
            }

+            // Clean up remaining state
            this.#loadedFonts.delete(key);
-            this.#usageTracker.delete(key);
            this.statuses.delete(key);
-            this.#retryCounts.delete(key);
+            this.#eviction.remove(key);
        }
    }

    /** Returns current loading status for a font, or undefined if never requested. */
    getFontStatus(id: string, weight: number, isVariable = false) {
-        return this.statuses.get(this.#getFontKey(id, weight, isVariable));
+        try {
+            return this.statuses.get(generateFontKey({ id, weight, isVariable }));
+        } catch (error) {
+            console.error(error);
+        }
    }

    /** Pins a font so it is never evicted by #purgeUnused(), regardless of TTL. */
-    pin(id: string, weight: number, isVariable?: boolean): void {
-        this.#pinnedFonts.add(this.#getFontKey(id, weight, !!isVariable));
+    pin(id: string, weight: number, isVariable = false): void {
+        this.#eviction.pin(generateFontKey({ id, weight, isVariable }));
    }

    /** Unpins a font, allowing it to be evicted by #purgeUnused() once its TTL expires. */
-    unpin(id: string, weight: number, isVariable?: boolean): void {
-        this.#pinnedFonts.delete(this.#getFontKey(id, weight, !!isVariable));
+    unpin(id: string, weight: number, isVariable = false): void {
+        this.#eviction.unpin(generateFontKey({ id, weight, isVariable }));
    }

    /** Waits for all fonts to finish loading using document.fonts.ready. */
    async ready(): Promise<void> {
-        if (typeof document === 'undefined') return;
+        if (typeof document === 'undefined') {
+            return;
+        }
        try {
            await document.fonts.ready;
-        } catch {
-            // document.fonts.ready can reject in some edge cases
-            // (e.g., document unloaded). Silently resolve.
-        }
+        } catch { /* document unloaded */ }
    }

    /** Aborts all operations, removes fonts from document, and clears state. Manager cannot be reused after. */
    destroy() {
+        // Abort all in-flight network requests
        this.#abortController.abort();

+        // Cancel pending queue processing (idle callback or timeout)
        if (this.#timeoutId !== null) {
            if (this.#pendingType === 'idle' && typeof cancelIdleCallback !== 'undefined') {
                cancelIdleCallback(this.#timeoutId as unknown as number);
@@ -365,25 +352,26 @@ export class AppliedFontsManager {
            this.#pendingType = null;
        }

+        // Stop periodic cleanup timer
        if (this.#intervalId) {
            clearInterval(this.#intervalId);
            this.#intervalId = null;
        }

+        // Remove all loaded fonts from document
        if (typeof document !== 'undefined') {
            for (const font of this.#loadedFonts.values()) {
                document.fonts.delete(font);
            }
        }

+        // Clear all state and collaborators
        this.#loadedFonts.clear();
-        this.#usageTracker.clear();
-        this.#retryCounts.clear();
-        this.#buffersByUrl.clear();
        this.#urlByKey.clear();
-        this.#pinnedFonts.clear();
-        this.statuses.clear();
+        this.#cache.clear();
+        this.#eviction.clear();
        this.#queue.clear();
+        this.statuses.clear();
    }
 }

--- a/src/entities/Font/model/store/appliedFontsStore/errors.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/errors.ts
@@ -0,0 +1,35 @@
+/**
+ * Thrown by {@link FontBufferCache} when a font file cannot be retrieved from the network or cache.
+ *
+ * @property url - The URL that was requested.
+ * @property cause - The underlying error, if any.
+ * @property status - HTTP status code. Present on HTTP errors, absent on network failures.
+ */
+export class FontFetchError extends Error {
+    readonly name = 'FontFetchError';
+
+    constructor(
+        public readonly url: string,
+        public readonly cause?: unknown,
+        public readonly status?: number,
+    ) {
+        super(status ? `HTTP ${status} fetching font: ${url}` : `Network error fetching font: ${url}`);
+    }
+}
+
+/**
+ * Thrown by {@link loadFont} when a font buffer cannot be parsed into a {@link FontFace}.
+ *
+ * @property fontName - The display name of the font that failed to parse.
+ * @property cause - The underlying error from the FontFace API.
+ */
+export class FontParseError extends Error {
+    readonly name = 'FontParseError';
+
+    constructor(
+        public readonly fontName: string,
+        public readonly cause?: unknown,
+    ) {
+        super(`Failed to parse font: ${fontName}`);
+    }
+}
--- a/src/entities/Font/model/store/appliedFontsStore/utils/fontBufferCache/FontBufferCache.test.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/fontBufferCache/FontBufferCache.test.ts
@@ -0,0 +1,66 @@
+/** @vitest-environment jsdom */
+import { FontFetchError } from '../../errors';
+import { FontBufferCache } from './FontBufferCache';
+
+const makeBuffer = () => new ArrayBuffer(8);
+
+const makeFetcher = (overrides: Partial<Response> = {}) =>
+    vi.fn().mockResolvedValue({
+        ok: true,
+        status: 200,
+        arrayBuffer: () => Promise.resolve(makeBuffer()),
+        clone: () => ({ ok: true, status: 200, arrayBuffer: () => Promise.resolve(makeBuffer()) }),
+        ...overrides,
+    } as Response);
+
+describe('FontBufferCache', () => {
+    let cache: FontBufferCache;
+    let fetcher: ReturnType<typeof makeFetcher>;
+
+    beforeEach(() => {
+        fetcher = makeFetcher();
+        cache = new FontBufferCache({ fetcher });
+    });
+
+    it('returns buffer from memory on second call without fetching', async () => {
+        await cache.get('https://example.com/font.woff2');
+        await cache.get('https://example.com/font.woff2');
+
+        expect(fetcher).toHaveBeenCalledOnce();
+    });
+
+    it('throws FontFetchError on HTTP error with correct status', async () => {
+        const errorFetcher = makeFetcher({ ok: false, status: 404 });
+        const errorCache = new FontBufferCache({ fetcher: errorFetcher });
+
+        const err = await errorCache.get('https://example.com/font.woff2').catch(e => e);
+        expect(err).toBeInstanceOf(FontFetchError);
+        expect(err.status).toBe(404);
+    });
+
+    it('throws FontFetchError on network failure without status', async () => {
+        const networkFetcher = vi.fn().mockRejectedValue(new Error('network down'));
+        const networkCache = new FontBufferCache({ fetcher: networkFetcher });
+
+        const err = await networkCache.get('https://example.com/font.woff2').catch(e => e);
+        expect(err).toBeInstanceOf(FontFetchError);
+        expect(err.status).toBeUndefined();
+    });
+
+    it('evict removes url from memory so next call fetches again', async () => {
+        await cache.get('https://example.com/font.woff2');
+        cache.evict('https://example.com/font.woff2');
+        await cache.get('https://example.com/font.woff2');
+
+        expect(fetcher).toHaveBeenCalledTimes(2);
+    });
+
+    it('clear wipes all memory cache entries', async () => {
+        await cache.get('https://example.com/a.woff2');
+        await cache.get('https://example.com/b.woff2');
+        cache.clear();
+        await cache.get('https://example.com/a.woff2');
+
+        expect(fetcher).toHaveBeenCalledTimes(3);
+    });
+});
--- a/src/entities/Font/model/store/appliedFontsStore/utils/fontBufferCache/FontBufferCache.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/fontBufferCache/FontBufferCache.ts
@@ -0,0 +1,97 @@
+import { FontFetchError } from '../../errors';
+
+type Fetcher = (url: string, init?: RequestInit) => Promise<Response>;
+
+interface FontBufferCacheOptions {
+    /** Custom fetch implementation. Defaults to `globalThis.fetch`. Inject in tests for isolation. */
+    fetcher?: Fetcher;
+    /** Cache API cache name. Defaults to `'font-cache-v1'`. */
+    cacheName?: string;
+}
+
+/**
+ * Three-tier font buffer cache: in-memory → Cache API → network.
+ *
+ * - **Tier 1 (memory):** Fastest — no I/O. Populated after first successful fetch.
+ * - **Tier 2 (Cache API):** Persists across page loads. Silently skipped in private browsing.
+ * - **Tier 3 (network):** Raw fetch. Throws {@link FontFetchError} on failure.
+ *
+ * The `fetcher` option is injectable for testing — pass a `vi.fn()` to avoid real network calls.
+ */
+export class FontBufferCache {
+    #buffersByUrl = new Map<string, ArrayBuffer>();
+
+    readonly #fetcher: Fetcher;
+    readonly #cacheName: string;
+
+    constructor(
+        { fetcher = globalThis.fetch.bind(globalThis), cacheName = 'font-cache-v1' }: FontBufferCacheOptions = {},
+    ) {
+        this.#fetcher = fetcher;
+        this.#cacheName = cacheName;
+    }
+
+    /**
+     * Retrieves the font buffer for the given URL using the three-tier strategy.
+     * Stores the result in memory on success.
+     *
+     * @throws {@link FontFetchError} if the network request fails or returns a non-OK response.
+     */
+    async get(url: string, signal?: AbortSignal): Promise<ArrayBuffer> {
+        // Tier 1: in-memory (fastest, no I/O)
+        const inMemory = this.#buffersByUrl.get(url);
+        if (inMemory) {
+            return inMemory;
+        }
+
+        // Tier 2: Cache API
+        try {
+            if (typeof caches !== 'undefined') {
+                const cache = await caches.open(this.#cacheName);
+                const cached = await cache.match(url);
+                if (cached) {
+                    const buffer = await cached.arrayBuffer();
+                    this.#buffersByUrl.set(url, buffer);
+                    return buffer;
+                }
+            }
+        } catch {
+            // Cache unavailable (private browsing, security restrictions) — fall through to network
+        }
+
+        // Tier 3: network
+        let response: Response;
+        try {
+            response = await this.#fetcher(url, { signal });
+        } catch (cause) {
+            throw new FontFetchError(url, cause);
+        }
+
+        if (!response.ok) {
+            throw new FontFetchError(url, undefined, response.status);
+        }
+
+        try {
+            if (typeof caches !== 'undefined') {
+                const cache = await caches.open(this.#cacheName);
+                await cache.put(url, response.clone());
+            }
+        } catch {
+            // Cache write failed (quota, storage pressure) — return font anyway
+        }
+
+        const buffer = await response.arrayBuffer();
+        this.#buffersByUrl.set(url, buffer);
+        return buffer;
+    }
+
+    /** Removes a URL from the in-memory cache. Next call to `get()` will re-fetch. */
+    evict(url: string): void {
+        this.#buffersByUrl.delete(url);
+    }
+
+    /** Clears all in-memory cached buffers. */
+    clear(): void {
+        this.#buffersByUrl.clear();
+    }
+}
--- a/src/entities/Font/model/store/appliedFontsStore/utils/fontEvictionPolicy/FontEvictionPolicy.test.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/fontEvictionPolicy/FontEvictionPolicy.test.ts
@@ -0,0 +1,69 @@
+import { FontEvictionPolicy } from './FontEvictionPolicy';
+
+describe('FontEvictionPolicy', () => {
+    let policy: FontEvictionPolicy;
+    const TTL = 1000;
+    const t0 = 100000;
+
+    beforeEach(() => {
+        policy = new FontEvictionPolicy({ ttl: TTL });
+    });
+
+    it('shouldEvict returns false within TTL', () => {
+        policy.touch('a@400', t0);
+        expect(policy.shouldEvict('a@400', t0 + TTL - 1)).toBe(false);
+    });
+
+    it('shouldEvict returns true at TTL boundary', () => {
+        policy.touch('a@400', t0);
+        expect(policy.shouldEvict('a@400', t0 + TTL)).toBe(true);
+    });
+
+    it('shouldEvict returns false for pinned key regardless of TTL', () => {
+        policy.touch('a@400', t0);
+        policy.pin('a@400');
+        expect(policy.shouldEvict('a@400', t0 + TTL * 10)).toBe(false);
+    });
+
+    it('shouldEvict returns true again after unpin past TTL', () => {
+        policy.touch('a@400', t0);
+        policy.pin('a@400');
+        policy.unpin('a@400');
+        expect(policy.shouldEvict('a@400', t0 + TTL)).toBe(true);
+    });
+
+    it('shouldEvict returns false for untracked key', () => {
+        expect(policy.shouldEvict('never@touched', t0 + TTL * 100)).toBe(false);
+    });
+
+    it('keys returns all tracked keys', () => {
+        policy.touch('a@400', t0);
+        policy.touch('b@vf', t0);
+        expect(Array.from(policy.keys())).toEqual(expect.arrayContaining(['a@400', 'b@vf']));
+    });
+
+    it('remove deletes key from tracking so it no longer appears in keys()', () => {
+        policy.touch('a@400', t0);
+        policy.touch('b@vf', t0);
+        policy.remove('a@400');
+        expect(Array.from(policy.keys())).not.toContain('a@400');
+        expect(Array.from(policy.keys())).toContain('b@vf');
+    });
+
+    it('remove unpins the key so a subsequent touch + TTL would evict it', () => {
+        policy.touch('a@400', t0);
+        policy.pin('a@400');
+        policy.remove('a@400');
+        // re-touch and check it can be evicted again
+        policy.touch('a@400', t0);
+        expect(policy.shouldEvict('a@400', t0 + TTL)).toBe(true);
+    });
+
+    it('clear resets all state', () => {
+        policy.touch('a@400', t0);
+        policy.pin('a@400');
+        policy.clear();
+        expect(Array.from(policy.keys())).toHaveLength(0);
+        expect(policy.shouldEvict('a@400', t0 + TTL * 10)).toBe(false);
+    });
+});
--- a/src/entities/Font/model/store/appliedFontsStore/utils/fontEvictionPolicy/FontEvictionPolicy.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/fontEvictionPolicy/FontEvictionPolicy.ts
@@ -0,0 +1,76 @@
+interface FontEvictionPolicyOptions {
+    /** TTL in milliseconds. Defaults to 5 minutes. */
+    ttl?: number;
+}
+
+/**
+ * Tracks font usage timestamps and pinned keys to determine when a font should be evicted.
+ *
+ * Pure data — no browser APIs. Accepts explicit `now` timestamps so tests
+ * never need fake timers.
+ */
+export class FontEvictionPolicy {
+    #usageTracker = new Map<string, number>();
+    #pinnedFonts = new Set<string>();
+
+    readonly #TTL: number;
+
+    constructor({ ttl = 5 * 60 * 1000 }: FontEvictionPolicyOptions = {}) {
+        this.#TTL = ttl;
+    }
+
+    /**
+     * Records the last-used time for a font key.
+     * @param key - Font key in `{id}@{weight}` or `{id}@vf` format.
+     * @param now - Current timestamp in ms. Defaults to `Date.now()`.
+     */
+    touch(key: string, now: number = Date.now()): void {
+        this.#usageTracker.set(key, now);
+    }
+
+    /** Pins a font key so it is never evicted regardless of TTL. */
+    pin(key: string): void {
+        this.#pinnedFonts.add(key);
+    }
+
+    /** Unpins a font key, allowing it to be evicted once its TTL expires. */
+    unpin(key: string): void {
+        this.#pinnedFonts.delete(key);
+    }
+
+    /**
+     * Returns `true` if the font should be evicted.
+     * A font is evicted when its TTL has elapsed and it is not pinned.
+     * Returns `false` for untracked keys.
+     *
+     * @param key - Font key to check.
+     * @param now - Current timestamp in ms (pass explicitly for deterministic tests).
+     */
+    shouldEvict(key: string, now: number): boolean {
+        const lastUsed = this.#usageTracker.get(key);
+        if (lastUsed === undefined) {
+            return false;
+        }
+        if (this.#pinnedFonts.has(key)) {
+            return false;
+        }
+        return now - lastUsed >= this.#TTL;
+    }
+
+    /** Returns an iterator over all tracked font keys. */
+    keys(): IterableIterator<string> {
+        return this.#usageTracker.keys();
+    }
+
+    /** Removes a font key from tracking. Called by the orchestrator after eviction. */
+    remove(key: string): void {
+        this.#usageTracker.delete(key);
+        this.#pinnedFonts.delete(key);
+    }
+
+    /** Clears all usage timestamps and pinned keys. */
+    clear(): void {
+        this.#usageTracker.clear();
+        this.#pinnedFonts.clear();
+    }
+}
--- a/src/entities/Font/model/store/appliedFontsStore/utils/fontLoadQueue/FontLoadQueue.test.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/fontLoadQueue/FontLoadQueue.test.ts
@@ -0,0 +1,65 @@
+import type { FontLoadRequestConfig } from '../../../../types';
+import { FontLoadQueue } from './FontLoadQueue';
+
+const config = (id: string): FontLoadRequestConfig => ({
+    id,
+    name: id,
+    url: `https://example.com/${id}.woff2`,
+    weight: 400,
+});
+
+describe('FontLoadQueue', () => {
+    let queue: FontLoadQueue;
+
+    beforeEach(() => {
+        queue = new FontLoadQueue();
+    });
+
+    it('enqueue returns true for a new key', () => {
+        expect(queue.enqueue('a@400', config('a'))).toBe(true);
+    });
+
+    it('enqueue returns false for an already-queued key', () => {
+        queue.enqueue('a@400', config('a'));
+        expect(queue.enqueue('a@400', config('a'))).toBe(false);
+    });
+
+    it('has returns true after enqueue, false after flush', () => {
+        queue.enqueue('a@400', config('a'));
+        expect(queue.has('a@400')).toBe(true);
+        queue.flush();
+        expect(queue.has('a@400')).toBe(false);
+    });
+
+    it('flush returns all entries and atomically clears the queue', () => {
+        queue.enqueue('a@400', config('a'));
+        queue.enqueue('b@700', config('b'));
+        const entries = queue.flush();
+        expect(entries).toHaveLength(2);
+        expect(queue.has('a@400')).toBe(false);
+        expect(queue.has('b@700')).toBe(false);
+    });
+
+    it('isMaxRetriesReached returns false below MAX_RETRIES', () => {
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        expect(queue.isMaxRetriesReached('a@400')).toBe(false);
+    });
+
+    it('isMaxRetriesReached returns true at MAX_RETRIES (3)', () => {
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        expect(queue.isMaxRetriesReached('a@400')).toBe(true);
+    });
+
+    it('clear resets queue and retry counts', () => {
+        queue.enqueue('a@400', config('a'));
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        queue.clear();
+        expect(queue.has('a@400')).toBe(false);
+        expect(queue.isMaxRetriesReached('a@400')).toBe(false);
+    });
+});
--- a/src/entities/Font/model/store/appliedFontsStore/utils/fontLoadQueue/FontLoadQueue.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/fontLoadQueue/FontLoadQueue.ts
@@ -0,0 +1,57 @@
+import type { FontLoadRequestConfig } from '../../../../types';
+
+/**
+ * Manages the font load queue and per-font retry counts.
+ *
+ * Scheduling (when to drain the queue) is handled by the orchestrator —
+ * this class is purely concerned with what is queued and whether retries are exhausted.
+ */
+export class FontLoadQueue {
+    #queue = new Map<string, FontLoadRequestConfig>();
+    #retryCounts = new Map<string, number>();
+
+    readonly #MAX_RETRIES = 3;
+
+    /**
+     * Adds a font to the queue.
+     * @returns `true` if the key was newly enqueued, `false` if it was already present.
+     */
+    enqueue(key: string, config: FontLoadRequestConfig): boolean {
+        if (this.#queue.has(key)) {
+            return false;
+        }
+        this.#queue.set(key, config);
+        return true;
+    }
+
+    /**
+     * Atomically snapshots and clears the queue.
+     * @returns All queued entries at the time of the call.
+     */
+    flush(): Array<[string, FontLoadRequestConfig]> {
+        const entries = Array.from(this.#queue.entries());
+        this.#queue.clear();
+        return entries;
+    }
+
+    /** Returns `true` if the key is currently in the queue. */
+    has(key: string): boolean {
+        return this.#queue.has(key);
+    }
+
+    /** Increments the retry count for a font key. */
+    incrementRetry(key: string): void {
+        this.#retryCounts.set(key, (this.#retryCounts.get(key) ?? 0) + 1);
+    }
+
+    /** Returns `true` if the font has reached or exceeded the maximum retry limit. */
+    isMaxRetriesReached(key: string): boolean {
+        return (this.#retryCounts.get(key) ?? 0) >= this.#MAX_RETRIES;
+    }
+
+    /** Clears all queued fonts and resets all retry counts. */
+    clear(): void {
+        this.#queue.clear();
+        this.#retryCounts.clear();
+    }
+}
--- a/src/entities/Font/model/store/appliedFontsStore/utils/generateFontKey/generateFontKey.test.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/generateFontKey/generateFontKey.test.ts
@@ -0,0 +1,25 @@
+import { generateFontKey } from './generateFontKey';
+
+describe('generateFontKey', () => {
+    it('should throw an error if font id is not provided', () => {
+        const config = { weight: 400, isVariable: false };
+        // @ts-expect-error
+        expect(() => generateFontKey(config)).toThrow('Font id is required');
+    });
+
+    it('should generate a font key for a variable font', () => {
+        const config = { id: 'Roboto', weight: 400, isVariable: true };
+        expect(generateFontKey(config)).toBe('roboto@vf');
+    });
+
+    it('should throw an error if font weight is not provided and is not a variable font', () => {
+        const config = { id: 'Roboto', isVariable: false };
+        // @ts-expect-error
+        expect(() => generateFontKey(config)).toThrow('Font weight is required');
+    });
+
+    it('should generate a font key for a non-variable font', () => {
+        const config = { id: 'Roboto', weight: 400, isVariable: false };
+        expect(generateFontKey(config)).toBe('roboto@400');
+    });
+});
--- a/src/entities/Font/model/store/appliedFontsStore/utils/generateFontKey/generateFontKey.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/generateFontKey/generateFontKey.ts
@@ -0,0 +1,22 @@
+import type { FontLoadRequestConfig } from '../../../../types';
+
+export type PartialConfig = Pick<FontLoadRequestConfig, 'id' | 'weight' | 'isVariable'>;
+
+/**
+ * Generates a font key for a given font load request configuration.
+ * @param config - The font load request configuration.
+ * @returns The generated font key.
+ */
+export function generateFontKey(config: PartialConfig): string {
+    if (!config.id) {
+        throw new Error('Font id is required');
+    }
+    if (config.isVariable) {
+        return `${config.id.toLowerCase()}@vf`;
+    }
+
+    if (!config.weight) {
+        throw new Error('Font weight is required');
+    }
+    return `${config.id.toLowerCase()}@${config.weight}`;
+}
--- a/src/entities/Font/model/store/appliedFontsStore/utils/getEffectiveConcurrency/getEffectiveConcurrency.test.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/getEffectiveConcurrency/getEffectiveConcurrency.test.ts
@@ -0,0 +1,41 @@
+import {
+    beforeEach,
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import {
+    Concurrency,
+    getEffectiveConcurrency,
+} from './getEffectiveConcurrency';
+
+describe('getEffectiveConcurrency', () => {
+    beforeEach(() => {
+        const nav = navigator as any;
+        nav.connection = null;
+    });
+
+    it('should return MAX when connection is not available', () => {
+        const nav = navigator as any;
+        nav.connection = null;
+        expect(getEffectiveConcurrency()).toBe(Concurrency.MAX);
+    });
+
+    it('should return MIN for slow-2g or 2g connection', () => {
+        const nav = navigator as any;
+        nav.connection = { effectiveType: 'slow-2g' };
+        expect(getEffectiveConcurrency()).toBe(Concurrency.MIN);
+    });
+
+    it('should return AVERAGE for 3g connection', () => {
+        const nav = navigator as any;
+        nav.connection = { effectiveType: '3g' };
+        expect(getEffectiveConcurrency()).toBe(Concurrency.AVERAGE);
+    });
+
+    it('should return MAX for other connection types', () => {
+        const nav = navigator as any;
+        nav.connection = { effectiveType: '4g' };
+        expect(getEffectiveConcurrency()).toBe(Concurrency.MAX);
+    });
+});
--- a/src/entities/Font/model/store/appliedFontsStore/utils/getEffectiveConcurrency/getEffectiveConcurrency.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/getEffectiveConcurrency/getEffectiveConcurrency.ts
@@ -0,0 +1,26 @@
+export enum Concurrency {
+    MIN = 1,
+    AVERAGE = 2,
+    MAX = 4,
+}
+
+/**
+ * Calculates the amount of fonts for concurrent download based on the user internet connection
+ */
+export function getEffectiveConcurrency(): number {
+    const nav = navigator as any;
+    const connection = nav.connection;
+    if (!connection) {
+        return Concurrency.MAX;
+    }
+
+    switch (connection.effectiveType) {
+        case 'slow-2g':
+        case '2g':
+            return Concurrency.MIN;
+        case '3g':
+            return Concurrency.AVERAGE;
+        default:
+            return Concurrency.MAX;
+    }
+}
--- a/src/entities/Font/model/store/appliedFontsStore/utils/index.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/index.ts
@@ -0,0 +1,4 @@
+export { generateFontKey } from './generateFontKey/generateFontKey';
+export { getEffectiveConcurrency } from './getEffectiveConcurrency/getEffectiveConcurrency';
+export { loadFont } from './loadFont/loadFont';
+export { yieldToMainThread } from './yieldToMainThread/yieldToMainThread';
--- a/src/entities/Font/model/store/appliedFontsStore/utils/loadFont/loadFont.test.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/loadFont/loadFont.test.ts
@@ -0,0 +1,93 @@
+/** @vitest-environment jsdom */
+import { FontParseError } from '../../errors';
+import { loadFont } from './loadFont';
+
+describe('loadFont', () => {
+    let mockFontInstance: any;
+    let mockFontFaceSet: { add: ReturnType<typeof vi.fn>; delete: ReturnType<typeof vi.fn> };
+
+    beforeEach(() => {
+        mockFontFaceSet = { add: vi.fn(), delete: vi.fn() };
+        Object.defineProperty(document, 'fonts', { value: mockFontFaceSet, configurable: true, writable: true });
+
+        const MockFontFace = vi.fn(
+            function(this: any, name: string, buffer: BufferSource, options: FontFaceDescriptors) {
+                this.name = name;
+                this.buffer = buffer;
+                this.options = options;
+                this.load = vi.fn().mockResolvedValue(this);
+                mockFontInstance = this;
+            },
+        );
+        vi.stubGlobal('FontFace', MockFontFace);
+    });
+
+    afterEach(() => {
+        vi.unstubAllGlobals();
+    });
+
+    it('constructs FontFace with exact weight for static fonts', async () => {
+        const buffer = new ArrayBuffer(8);
+        await loadFont({ name: 'Roboto', weight: 400 }, buffer);
+
+        expect(FontFace).toHaveBeenCalledWith('Roboto', buffer, expect.objectContaining({ weight: '400' }));
+    });
+
+    it('constructs FontFace with weight range for variable fonts', async () => {
+        const buffer = new ArrayBuffer(8);
+        await loadFont({ name: 'Roboto', weight: 400, isVariable: true }, buffer);
+
+        expect(FontFace).toHaveBeenCalledWith('Roboto', buffer, expect.objectContaining({ weight: '100 900' }));
+    });
+
+    it('sets style: normal and display: swap on FontFace options', async () => {
+        await loadFont({ name: 'Lato', weight: 700 }, new ArrayBuffer(8));
+
+        expect(FontFace).toHaveBeenCalledWith(
+            'Lato',
+            expect.anything(),
+            expect.objectContaining({ style: 'normal', display: 'swap' }),
+        );
+    });
+
+    it('passes the buffer as the second argument to FontFace', async () => {
+        const buffer = new ArrayBuffer(16);
+        await loadFont({ name: 'Inter', weight: 400 }, buffer);
+
+        expect(FontFace).toHaveBeenCalledWith('Inter', buffer, expect.anything());
+    });
+
+    it('calls font.load() and adds the font to document.fonts', async () => {
+        const buffer = new ArrayBuffer(8);
+        const result = await loadFont({ name: 'Inter', weight: 400 }, buffer);
+
+        expect(mockFontInstance.load).toHaveBeenCalledOnce();
+        expect(mockFontFaceSet.add).toHaveBeenCalledWith(mockFontInstance);
+        expect(result).toBe(mockFontInstance);
+    });
+
+    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) {
+                this.load = vi.fn().mockRejectedValue(loadError);
+            },
+        );
+        vi.stubGlobal('FontFace', MockFontFace);
+
+        await expect(loadFont({ name: 'Broken', weight: 400 }, new ArrayBuffer(8))).rejects.toBeInstanceOf(
+            FontParseError,
+        );
+    });
+
+    it('throws FontParseError when document.fonts.add throws', async () => {
+        const addError = new Error('add failed');
+        mockFontFaceSet.add.mockImplementation(() => {
+            throw addError;
+        });
+
+        await expect(loadFont({ name: 'Broken', weight: 400 }, new ArrayBuffer(8))).rejects.toBeInstanceOf(
+            FontParseError,
+        );
+    });
+});
--- a/src/entities/Font/model/store/appliedFontsStore/utils/loadFont/loadFont.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/loadFont/loadFont.ts
@@ -0,0 +1,27 @@
+import type { FontLoadRequestConfig } from '../../../../types';
+import { FontParseError } from '../../errors';
+
+export type PartialConfig = Pick<FontLoadRequestConfig, 'weight' | 'name' | 'isVariable'>;
+/**
+ * Loads a font from a buffer and adds it to the document's font collection.
+ * @param config - The font load request configuration.
+ * @param buffer - The buffer containing the font data.
+ * @returns A promise that resolves to the loaded `FontFace`.
+ * @throws {@link FontParseError} When the font buffer cannot be parsed or added to the document font set.
+ */
+export async function loadFont(config: PartialConfig, buffer: BufferSource): Promise<FontFace> {
+    try {
+        const weightRange = config.isVariable ? '100 900' : `${config.weight}`;
+        const font = new FontFace(config.name, buffer, {
+            weight: weightRange,
+            style: 'normal',
+            display: 'swap',
+        });
+        await font.load();
+        document.fonts.add(font);
+
+        return font;
+    } catch (error) {
+        throw new FontParseError(config.name, error);
+    }
+}
--- a/src/entities/Font/model/store/appliedFontsStore/utils/yieldToMainThread/yieldToMainThread.test.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/yieldToMainThread/yieldToMainThread.test.ts
@@ -0,0 +1,17 @@
+import { yieldToMainThread } from './yieldToMainThread';
+
+describe('yieldToMainThread', () => {
+    it('uses scheduler.yield when available', async () => {
+        const mockYield = vi.fn().mockResolvedValue(undefined);
+        vi.stubGlobal('scheduler', { yield: mockYield });
+
+        await yieldToMainThread();
+
+        expect(mockYield).toHaveBeenCalledOnce();
+        vi.unstubAllGlobals();
+    });
+    it('falls back to MessageChannel when scheduler is unavailable', async () => {
+        // scheduler is not defined in jsdom by default
+        await expect(yieldToMainThread()).resolves.toBeUndefined();
+    });
+});
--- a/src/entities/Font/model/store/appliedFontsStore/utils/yieldToMainThread/yieldToMainThread.ts
+++ b/src/entities/Font/model/store/appliedFontsStore/utils/yieldToMainThread/yieldToMainThread.ts
@@ -0,0 +1,16 @@
+/**
+ * Yields to main thread during CPU-intensive parsing. Uses scheduler.yield() where available or MessageChannel fallback.
+ */
+export async function yieldToMainThread(): Promise<void> {
+    // @ts-expect-error - scheduler not in TypeScript lib yet
+    if (typeof scheduler !== 'undefined' && 'yield' in scheduler) {
+        // @ts-expect-error - scheduler.yield not in TypeScript lib yet
+        await scheduler.yield();
+    } else {
+        await new Promise<void>(resolve => {
+            const ch = new MessageChannel();
+            ch.port1.onmessage = () => resolve();
+            ch.port2.postMessage(null);
+        });
+    }
+}
--- a/src/entities/Font/model/store/baseFontStore/baseFontStore.svelte.spec.ts
+++ b/src/entities/Font/model/store/baseFontStore/baseFontStore.svelte.spec.ts
@@ -0,0 +1,644 @@
+import { QueryClient } from '@tanstack/query-core';
+import { flushSync } from 'svelte';
+import {
+    afterEach,
+    beforeEach,
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+import { generateMockFonts } from '../../../lib/mocks/fonts.mock';
+import type { UnifiedFont } from '../../types';
+import { BaseFontStore } from './baseFontStore.svelte';
+
+vi.mock('$shared/api/queryClient', () => ({
+    queryClient: new QueryClient({
+        defaultOptions: {
+            queries: {
+                retry: 0,
+                gcTime: 0,
+            },
+        },
+    }),
+}));
+
+import { queryClient } from '$shared/api/queryClient';
+
+interface TestParams {
+    limit?: number;
+    offset?: number;
+    q?: string;
+    providers?: string[];
+    categories?: string[];
+    subsets?: string[];
+}
+
+class TestFontStore extends BaseFontStore<TestParams> {
+    protected getQueryKey(params: TestParams) {
+        return ['testFonts', params] as const;
+    }
+
+    protected async fetchFn(params: TestParams): Promise<UnifiedFont[]> {
+        return generateMockFonts(params.limit || 10);
+    }
+}
+
+describe('baseFontStore', () => {
+    describe('constructor', () => {
+        afterEach(() => {
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('creates a new store with initial params', () => {
+            const store = new TestFontStore({ limit: 20, offset: 10 });
+
+            expect(store.params.limit).toBe(20);
+            expect(store.params.offset).toBe(10);
+            store.destroy();
+        });
+
+        it('defaults offset to 0 if not provided', () => {
+            const store = new TestFontStore({ limit: 10 });
+
+            expect(store.params.offset).toBe(0);
+            store.destroy();
+        });
+
+        it('initializes observer with query options', () => {
+            const store = new TestFontStore({ limit: 10 });
+
+            expect((store as any).observer).toBeDefined();
+            store.destroy();
+        });
+    });
+
+    describe('params getter', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10, offset: 0 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('returns merged internal params', () => {
+            store.setParams({ limit: 20 });
+            flushSync();
+
+            expect(store.params.limit).toBe(20);
+            expect(store.params.offset).toBe(0);
+        });
+
+        it('defaults offset to 0 when undefined', () => {
+            const store2 = new TestFontStore({});
+            flushSync();
+
+            expect(store2.params.offset).toBe(0);
+            store2.destroy();
+        });
+    });
+
+    describe('state getters', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        describe('fonts', () => {
+            it('returns fonts after auto-fetch on mount', async () => {
+                await new Promise(resolve => setTimeout(resolve, 0));
+
+                expect(store.fonts).toHaveLength(10);
+            });
+
+            it('returns fonts when data is loaded', async () => {
+                await store.refetch();
+                flushSync();
+
+                expect(store.fonts).toHaveLength(10);
+            });
+
+            it('returns fonts when data is loaded', async () => {
+                await store.refetch();
+                flushSync();
+
+                expect(store.fonts).toHaveLength(10);
+            });
+        });
+
+        describe('isLoading', () => {
+            it('is false after initial fetch completes', async () => {
+                await store.refetch();
+                flushSync();
+
+                expect(store.isLoading).toBe(false);
+            });
+
+            it('is false when error occurs', async () => {
+                vi.spyOn(store, 'fetchFn' as any).mockRejectedValue(new Error('fail'));
+                await store.refetch().catch(() => {});
+                flushSync();
+
+                expect(store.isLoading).toBe(false);
+            });
+        });
+
+        describe('isFetching', () => {
+            it('is false after fetch completes', async () => {
+                await store.refetch();
+                flushSync();
+
+                expect(store.isFetching).toBe(false);
+            });
+
+            it('is true during refetch', async () => {
+                await store.refetch();
+                flushSync();
+
+                const refetchPromise = store.refetch();
+                flushSync();
+
+                expect(store.isFetching).toBe(true);
+                await refetchPromise;
+            });
+        });
+
+        describe('isError', () => {
+            it('is false initially', () => {
+                expect(store.isError).toBe(false);
+            });
+
+            it('is true after fetch error', async () => {
+                vi.spyOn(store, 'fetchFn' as any).mockRejectedValue(new Error('fail'));
+                await store.refetch().catch(() => {});
+                flushSync();
+
+                expect(store.isError).toBe(true);
+            });
+
+            it('is false after successful fetch', async () => {
+                await store.refetch();
+                flushSync();
+
+                expect(store.isError).toBe(false);
+            });
+        });
+
+        describe('error', () => {
+            it('is null initially', () => {
+                expect(store.error).toBeNull();
+            });
+
+            it('returns error object after fetch error', async () => {
+                const testError = new Error('test error');
+                vi.spyOn(store, 'fetchFn' as any).mockRejectedValue(testError);
+                await store.refetch().catch(() => {});
+                flushSync();
+
+                expect(store.error).toBe(testError);
+            });
+
+            it('is null after successful fetch', async () => {
+                await store.refetch();
+                flushSync();
+
+                expect(store.error).toBeNull();
+            });
+        });
+
+        describe('isEmpty', () => {
+            it('is true when no fonts loaded and not loading', async () => {
+                await store.refetch();
+                flushSync();
+                store.setQueryData(() => []);
+                flushSync();
+
+                expect(store.isEmpty).toBe(true);
+            });
+
+            it('is false when fonts are present', async () => {
+                await store.refetch();
+                flushSync();
+
+                expect(store.isEmpty).toBe(false);
+            });
+
+            it('is false when loading', () => {
+                expect(store.isEmpty).toBe(false);
+            });
+        });
+    });
+
+    describe('setParams', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10, offset: 0 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('merges new params with existing', () => {
+            store.setParams({ limit: 20 });
+            flushSync();
+
+            expect(store.params.limit).toBe(20);
+            expect(store.params.offset).toBe(0);
+        });
+
+        it('replaces existing param values', () => {
+            store.setParams({ limit: 30 });
+            flushSync();
+
+            store.setParams({ limit: 40 });
+            flushSync();
+
+            expect(store.params.limit).toBe(40);
+        });
+
+        it('triggers observer options update', async () => {
+            const setOptionsSpy = vi.spyOn((store as any).observer, 'setOptions');
+
+            store.setParams({ limit: 20 });
+            flushSync();
+
+            expect(setOptionsSpy).toHaveBeenCalled();
+        });
+    });
+
+    describe('updateInternalParams', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10, offset: 20 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('updates internal params without triggering setParams hooks', () => {
+            (store as any).updateInternalParams({ offset: 0 });
+            flushSync();
+
+            expect(store.params.offset).toBe(0);
+            expect(store.params.limit).toBe(10);
+        });
+
+        it('merges with existing internal params', () => {
+            (store as any).updateInternalParams({ offset: 0, limit: 30 });
+            flushSync();
+
+            expect(store.params.offset).toBe(0);
+            expect(store.params.limit).toBe(30);
+        });
+
+        it('updates observer options', () => {
+            const setOptionsSpy = vi.spyOn((store as any).observer, 'setOptions');
+
+            (store as any).updateInternalParams({ offset: 0 });
+            flushSync();
+
+            expect(setOptionsSpy).toHaveBeenCalled();
+        });
+    });
+
+    describe('invalidate', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('invalidates query for current params', async () => {
+            await store.refetch();
+            flushSync();
+
+            const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries');
+            store.invalidate();
+
+            expect(invalidateSpy).toHaveBeenCalledWith({
+                queryKey: ['testFonts', store.params],
+            });
+        });
+
+        it('triggers refetch of invalidated query', async () => {
+            await store.refetch();
+            flushSync();
+
+            const fetchSpy = vi.spyOn(store, 'fetchFn' as any);
+            store.invalidate();
+            await store.refetch();
+            flushSync();
+
+            expect(fetchSpy).toHaveBeenCalled();
+        });
+    });
+
+    describe('destroy', () => {
+        it('calls cleanup function', () => {
+            const store = new TestFontStore({ limit: 10 });
+            const cleanupSpy = vi.spyOn(store, 'cleanup' as any);
+
+            store.destroy();
+
+            expect(cleanupSpy).toHaveBeenCalled();
+        });
+
+        it('can be called multiple times without error', () => {
+            const store = new TestFontStore({ limit: 10 });
+
+            expect(() => {
+                store.destroy();
+                store.destroy();
+            }).not.toThrow();
+        });
+    });
+
+    describe('refetch', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('triggers a refetch', async () => {
+            const fetchSpy = vi.spyOn(store, 'fetchFn' as any);
+            await store.refetch();
+            flushSync();
+
+            expect(fetchSpy).toHaveBeenCalled();
+        });
+
+        it('updates observer options before refetching', async () => {
+            const setOptionsSpy = vi.spyOn((store as any).observer, 'setOptions');
+            const refetchSpy = vi.spyOn((store as any).observer, 'refetch');
+
+            await store.refetch();
+            flushSync();
+
+            expect(setOptionsSpy).toHaveBeenCalledBefore(refetchSpy);
+        });
+
+        it('uses current params for refetch', async () => {
+            store.setParams({ limit: 20 });
+            flushSync();
+
+            await store.refetch();
+            flushSync();
+
+            expect(store.params.limit).toBe(20);
+        });
+    });
+
+    describe('prefetch', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('prefetches data with provided params', async () => {
+            const prefetchSpy = vi.spyOn(queryClient, 'prefetchQuery');
+
+            await store.prefetch({ limit: 20, offset: 0 });
+
+            expect(prefetchSpy).toHaveBeenCalled();
+        });
+
+        it('stores prefetched data in cache', async () => {
+            queryClient.clear();
+
+            const store2 = new TestFontStore({ limit: 10 });
+            await store2.prefetch({ limit: 5, offset: 0 });
+            flushSync();
+
+            const cached = store2.getCachedData();
+            expect(cached).toBeDefined();
+            expect(cached?.length).toBeGreaterThanOrEqual(0);
+            store2.destroy();
+        });
+    });
+
+    describe('cancel', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('cancels ongoing queries', () => {
+            const cancelSpy = vi.spyOn(queryClient, 'cancelQueries');
+
+            store.cancel();
+
+            expect(cancelSpy).toHaveBeenCalledWith({
+                queryKey: ['testFonts', store.params],
+            });
+        });
+    });
+
+    describe('getCachedData', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('returns undefined when no data cached', () => {
+            queryClient.clear();
+
+            const store2 = new TestFontStore({ limit: 10 });
+            expect(store2.getCachedData()).toBeUndefined();
+            store2.destroy();
+        });
+
+        it('returns cached data after fetch', async () => {
+            await store.refetch();
+            flushSync();
+
+            const cached = store.getCachedData();
+            expect(cached).toHaveLength(10);
+        });
+
+        it('returns data from manual cache update', () => {
+            store.setQueryData(() => [generateMockFonts(1)[0]]);
+            flushSync();
+
+            const cached = store.getCachedData();
+            expect(cached).toHaveLength(1);
+        });
+    });
+
+    describe('setQueryData', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('sets data in cache', () => {
+            store.setQueryData(() => [generateMockFonts(1)[0]]);
+            flushSync();
+
+            const cached = store.getCachedData();
+            expect(cached).toHaveLength(1);
+        });
+
+        it('updates existing cached data', async () => {
+            await store.refetch();
+            flushSync();
+
+            store.setQueryData(old => [...(old || []), generateMockFonts(1)[0]]);
+            flushSync();
+
+            const cached = store.getCachedData();
+            expect(cached).toHaveLength(11);
+        });
+
+        it('receives previous data in updater function', async () => {
+            await store.refetch();
+            flushSync();
+
+            const updater = vi.fn((old: UnifiedFont[] | undefined) => old || []);
+            store.setQueryData(updater);
+            flushSync();
+
+            expect(updater).toHaveBeenCalledWith(expect.any(Array));
+        });
+    });
+
+    describe('getOptions', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('returns query options with query key', () => {
+            const options = (store as any).getOptions();
+
+            expect(options.queryKey).toEqual(['testFonts', store.params]);
+        });
+
+        it('returns query options with query fn', () => {
+            const options = (store as any).getOptions();
+
+            expect(options.queryFn).toBeDefined();
+        });
+
+        it('uses provided params when passed', () => {
+            const customParams = { limit: 20, offset: 0 };
+            const options = (store as any).getOptions(customParams);
+
+            expect(options.queryKey).toEqual(['testFonts', customParams]);
+        });
+
+        it('has default staleTime and gcTime', () => {
+            const options = (store as any).getOptions();
+
+            expect(options.staleTime).toBe(5 * 60 * 1000);
+            expect(options.gcTime).toBe(10 * 60 * 1000);
+        });
+    });
+
+    describe('observer integration', () => {
+        let store: TestFontStore;
+
+        beforeEach(() => {
+            store = new TestFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('syncs observer state to Svelte state', async () => {
+            await store.refetch();
+            flushSync();
+
+            expect(store.fonts).toHaveLength(10);
+        });
+
+        it('observer syncs on state changes', async () => {
+            await store.refetch();
+            flushSync();
+
+            expect((store as any).result.data).toHaveLength(10);
+        });
+    });
+
+    describe('effect cleanup', () => {
+        it('cleanup function is set on constructor', () => {
+            const store = new TestFontStore({ limit: 10 });
+
+            expect(store.cleanup).toBeDefined();
+            expect(typeof store.cleanup).toBe('function');
+
+            store.destroy();
+        });
+    });
+});
--- a/src/entities/Font/model/store/baseFontStore/baseFontStore.svelte.ts
+++ b/src/entities/Font/model/store/baseFontStore/baseFontStore.svelte.ts
@@ -5,7 +5,7 @@ import {
    type QueryObserverOptions,
    type QueryObserverResult,
 } from '@tanstack/query-core';
-import type { UnifiedFont } from '../types';
+import type { UnifiedFont } from '../../types';

 /**
 * Base class for font stores using TanStack Query
@@ -23,25 +23,22 @@ export abstract class BaseFontStore<TParams extends Record<string, any>> {
     */
    cleanup: () => void;

-    /** Reactive parameter bindings from external sources */
-    #bindings = $state<(() => Partial<TParams>)[]>([]);
    /** Internal parameter state */
    #internalParams = $state<TParams>({} as TParams);

    /**
-     * Merged params from internal state and all bindings
-     * Automatically updates when bindings or internal params change
+     * Merged params from internal state
+     * Computed synchronously on access
     */
-    params = $derived.by(() => {
-        let merged = { ...this.#internalParams };
-
-        // Merge all binding results into params
-        for (const getter of this.#bindings) {
-            const bindingResult = getter();
-            merged = { ...merged, ...bindingResult };
+    get params(): TParams {
+        // Default offset to 0 if undefined (for pagination methods)
+        let result = this.#internalParams as TParams;
+        if (result.offset === undefined) {
+            result = { ...result, offset: 0 } as TParams;
        }
-        return merged as TParams;
-    });
+
+        return result;
+    }

    /** TanStack Query result state */
    protected result = $state<QueryObserverResult<UnifiedFont[], Error>>({} as any);
@@ -89,9 +86,10 @@ export abstract class BaseFontStore<TParams extends Record<string, any>> {
     * @param params - Query parameters (defaults to current params)
     */
    protected getOptions(params = this.params): QueryObserverOptions<UnifiedFont[], Error> {
+        // Always use current params, not the captured closure params
        return {
            queryKey: this.getQueryKey(params),
-            queryFn: () => this.fetchFn(params),
+            queryFn: () => this.fetchFn(this.params),
            staleTime: 5 * 60 * 1000,
            gcTime: 10 * 60 * 1000,
        };
@@ -117,30 +115,35 @@ export abstract class BaseFontStore<TParams extends Record<string, any>> {
        return this.result.isError;
    }

+    /** The error from the last failed fetch, or null if no error. */
+    get error(): Error | null {
+        return this.result.error ?? null;
+    }
+
    /** Whether no fonts are loaded (not loading and empty array) */
    get isEmpty() {
        return !this.isLoading && this.fonts.length === 0;
    }

-    /**
-     * Add a reactive parameter binding
-     * @param getter - Function that returns partial params to merge
-     * @returns Unbind function to remove the binding
-     */
-    addBinding(getter: () => Partial<TParams>) {
-        this.#bindings.push(getter);
-
-        return () => {
-            this.#bindings = this.#bindings.filter(b => b !== getter);
-        };
-    }
-
    /**
     * Update query parameters
     * @param newParams - Partial params to merge with existing
     */
    setParams(newParams: Partial<TParams>) {
-        this.#internalParams = { ...this.params, ...newParams };
+        this.#internalParams = { ...this.#internalParams, ...newParams };
+        // Manually update observer options since effects may not run in test contexts
+        this.observer.setOptions(this.getOptions());
+    }
+
+    /**
+     * Update internal params without triggering setParams hooks
+     * Used for resetting offset when filters change
+     * @param newParams - Partial params to merge with existing
+     */
+    protected updateInternalParams(newParams: Partial<TParams>) {
+        this.#internalParams = { ...this.#internalParams, ...newParams };
+        // Update observer options
+        this.observer.setOptions(this.getOptions());
    }

    /**
@@ -161,6 +164,8 @@ export abstract class BaseFontStore<TParams extends Record<string, any>> {
     * Manually trigger a refetch
     */
    async refetch() {
+        // Update options before refetching to ensure current params are used
+        this.observer.setOptions(this.getOptions());
        await this.observer.refetch();
    }

@@ -180,15 +185,6 @@ export abstract class BaseFontStore<TParams extends Record<string, any>> {
        });
    }

-    /**
-     * Clear cache for current params
-     */
-    clearCache() {
-        this.qc.removeQueries({
-            queryKey: this.getQueryKey(this.params),
-        });
-    }
-
    /**
     * Get cached data without triggering fetch
     */
--- a/src/entities/Font/model/store/index.ts
+++ b/src/entities/Font/model/store/index.ts
@@ -11,10 +11,7 @@ export {
    createUnifiedFontStore,
    type UnifiedFontStore,
    unifiedFontStore,
-} from './unifiedFontStore.svelte';
+} from './unifiedFontStore/unifiedFontStore.svelte';

 // Applied fonts manager (CSS loading - unchanged)
-export {
-    appliedFontsManager,
-    type FontConfigRequest,
-} from './appliedFontsStore/appliedFontsStore.svelte';
+export { appliedFontsManager } from './appliedFontsStore/appliedFontsStore.svelte';
--- a/src/entities/Font/model/store/unifiedFontStore/unifiedFontStore.svelte.spec.ts
+++ b/src/entities/Font/model/store/unifiedFontStore/unifiedFontStore.svelte.spec.ts
@@ -0,0 +1,474 @@
+import { QueryClient } from '@tanstack/query-core';
+import { tick } from 'svelte';
+import {
+    afterEach,
+    beforeEach,
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+import {
+    FontNetworkError,
+    FontResponseError,
+} from '../../../lib/errors/errors';
+
+vi.mock('$shared/api/queryClient', () => ({
+    queryClient: new QueryClient({
+        defaultOptions: {
+            queries: {
+                retry: 0,
+                gcTime: 0,
+            },
+        },
+    }),
+}));
+
+vi.mock('../../../api', () => ({
+    fetchProxyFonts: vi.fn(),
+}));
+
+import { queryClient } from '$shared/api/queryClient';
+import { flushSync } from 'svelte';
+import { fetchProxyFonts } from '../../../api';
+import {
+    generateMixedCategoryFonts,
+    generateMockFonts,
+} from '../../../lib/mocks/fonts.mock';
+import type { UnifiedFont } from '../../types';
+import { UnifiedFontStore } from './unifiedFontStore.svelte';
+
+const mockedFetch = fetchProxyFonts as ReturnType<typeof vi.fn>;
+
+const makeResponse = (
+    fonts: UnifiedFont[],
+    meta: { total?: number; limit?: number; offset?: number } = {},
+) => ({
+    fonts,
+    total: meta.total ?? fonts.length,
+    limit: meta.limit ?? 10,
+    offset: meta.offset ?? 0,
+});
+describe('unifiedFontStore', () => {
+    describe('fetchFn — error paths', () => {
+        let store: UnifiedFontStore;
+
+        beforeEach(() => {
+            store = new UnifiedFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('sets isError and error getter when fetchProxyFonts throws', async () => {
+            mockedFetch.mockRejectedValue(new Error('network down'));
+            await store.refetch().catch((e: unknown) => e);
+
+            expect(store.error).toBeInstanceOf(FontNetworkError);
+            expect((store.error as FontNetworkError).cause).toBeInstanceOf(Error);
+            expect(store.isError).toBe(true);
+        });
+
+        it('throws FontResponseError when response is falsy', async () => {
+            mockedFetch.mockResolvedValue(undefined);
+
+            await store.refetch().catch((e: unknown) => e);
+
+            expect(store.error).toBeInstanceOf(FontResponseError);
+            expect((store.error as FontResponseError).field).toBe('response');
+        });
+
+        it('throws FontResponseError when response.fonts is missing', async () => {
+            mockedFetch.mockResolvedValue({ total: 0, limit: 10, offset: 0 });
+
+            await store.refetch().catch((e: unknown) => e);
+
+            expect(store.error).toBeInstanceOf(FontResponseError);
+            expect((store.error as FontResponseError).field).toBe('response.fonts');
+        });
+
+        it('throws FontResponseError when response.fonts is not an array', async () => {
+            mockedFetch.mockResolvedValue({ fonts: 'bad', total: 0, limit: 10, offset: 0 });
+
+            await store.refetch().catch((e: unknown) => e);
+
+            expect(store.error).toBeInstanceOf(FontResponseError);
+            expect((store.error as FontResponseError).field).toBe('response.fonts');
+            expect((store.error as FontResponseError).received).toBe('bad');
+        });
+    });
+
+    describe('fetchFn — success path', () => {
+        let store: UnifiedFontStore;
+
+        beforeEach(() => {
+            store = new UnifiedFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('populates fonts after a successful fetch', async () => {
+            const fonts = generateMockFonts(3);
+            mockedFetch.mockResolvedValue(makeResponse(fonts));
+            await store.refetch();
+
+            expect(store.fonts).toHaveLength(3);
+            expect(store.fonts[0].id).toBe(fonts[0].id);
+        });
+
+        it('stores pagination metadata from response', async () => {
+            const fonts = generateMockFonts(3);
+            mockedFetch.mockResolvedValue(makeResponse(fonts, { total: 30, limit: 10, offset: 0 }));
+            await store.refetch();
+
+            expect(store.pagination.total).toBe(30);
+            expect(store.pagination.limit).toBe(10);
+            expect(store.pagination.offset).toBe(0);
+        });
+
+        it('replaces accumulated fonts on offset-0 fetch', async () => {
+            const first = generateMockFonts(3);
+            mockedFetch.mockResolvedValue(makeResponse(first));
+            await store.refetch();
+            flushSync();
+
+            const second = generateMockFonts(2);
+            mockedFetch.mockResolvedValue(makeResponse(second));
+            await store.refetch();
+            flushSync();
+
+            expect(store.fonts).toHaveLength(2);
+            expect(store.fonts[0].id).toBe(second[0].id);
+        });
+
+        it('appends fonts when fetching at offset > 0', async () => {
+            const firstPage = generateMockFonts(3);
+            mockedFetch.mockResolvedValue(makeResponse(firstPage, { total: 6, limit: 3, offset: 0 }));
+            await store.refetch();
+
+            const secondPage = generateMockFonts(3).map((f, i) => ({
+                ...f,
+                id: `page2-font-${i + 1}`,
+            }));
+            mockedFetch.mockResolvedValue(makeResponse(secondPage, { total: 6, limit: 3, offset: 3 }));
+            store.setParams({ offset: 3 });
+            await store.refetch();
+
+            expect(store.fonts).toHaveLength(6);
+            expect(store.fonts.slice(0, 3).map(f => f.id)).toEqual(firstPage.map(f => f.id));
+            expect(store.fonts.slice(3).map(f => f.id)).toEqual(secondPage.map(f => f.id));
+        });
+    });
+
+    describe('pagination state', () => {
+        let store: UnifiedFontStore;
+
+        beforeEach(() => {
+            store = new UnifiedFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('returns default pagination before any fetch', () => {
+            expect(store.pagination.total).toBe(0);
+            expect(store.pagination.hasMore).toBe(false);
+            expect(store.pagination.page).toBe(1);
+            expect(store.pagination.totalPages).toBe(0);
+        });
+
+        it('computes hasMore as true when more pages remain', async () => {
+            mockedFetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 30, limit: 10, offset: 0 }));
+            await store.refetch();
+
+            expect(store.pagination.hasMore).toBe(true);
+        });
+
+        it('computes hasMore as false on last page', async () => {
+            mockedFetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 20, limit: 10, offset: 10 }));
+            store.setParams({ offset: 10 });
+            await store.refetch();
+
+            expect(store.pagination.hasMore).toBe(false);
+        });
+
+        it('computes page and totalPages from response metadata', async () => {
+            mockedFetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 30, limit: 10, offset: 10 }));
+            store.setParams({ offset: 10 });
+            await store.refetch();
+
+            expect(store.pagination.page).toBe(2);
+            expect(store.pagination.totalPages).toBe(3);
+        });
+    });
+
+    describe('pagination navigation', () => {
+        let store: UnifiedFontStore;
+
+        beforeEach(async () => {
+            mockedFetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 30, limit: 10, offset: 0 }));
+            store = new UnifiedFontStore({ limit: 10 });
+            await tick();
+            await store.refetch();
+            await tick();
+            flushSync();
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('nextPage() advances offset by limit when hasMore', () => {
+            store.nextPage();
+            flushSync();
+
+            expect(store.params.offset).toBe(10);
+        });
+
+        it('nextPage() does nothing when hasMore is false', async () => {
+            mockedFetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 20, limit: 10, offset: 10 }));
+            store.setParams({ offset: 10 });
+            await store.refetch();
+            flushSync();
+
+            store.nextPage();
+            flushSync();
+
+            expect(store.params.offset).toBe(10);
+        });
+
+        it('prevPage() decrements offset by limit when on page > 1', async () => {
+            mockedFetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 30, limit: 10, offset: 10 }));
+            store.setParams({ offset: 10 });
+            await store.refetch();
+            flushSync();
+
+            store.prevPage();
+            flushSync();
+
+            expect(store.params.offset).toBe(0);
+        });
+
+        it('prevPage() does nothing on the first page', () => {
+            store.prevPage();
+            flushSync();
+
+            expect(store.params.offset).toBe(0);
+        });
+
+        it('goToPage() sets the correct offset', () => {
+            store.goToPage(2);
+            flushSync();
+
+            expect(store.params.offset).toBe(10);
+        });
+
+        it('goToPage() does nothing for page 0', () => {
+            store.goToPage(0);
+            flushSync();
+
+            expect(store.params.offset).toBe(0);
+        });
+
+        it('goToPage() does nothing for page beyond totalPages', () => {
+            store.goToPage(99);
+            flushSync();
+
+            expect(store.params.offset).toBe(0);
+        });
+
+        it('setLimit() updates the limit param', () => {
+            store.setLimit(25);
+            flushSync();
+
+            expect(store.params.limit).toBe(25);
+        });
+    });
+
+    describe('filter setters', () => {
+        let store: UnifiedFontStore;
+
+        beforeEach(() => {
+            store = new UnifiedFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('setProviders() updates the providers param', () => {
+            store.setProviders(['google']);
+            flushSync();
+
+            expect(store.params.providers).toEqual(['google']);
+        });
+
+        it('setCategories() updates the categories param', () => {
+            store.setCategories(['serif']);
+            flushSync();
+
+            expect(store.params.categories).toEqual(['serif']);
+        });
+
+        it('setSubsets() updates the subsets param', () => {
+            store.setSubsets(['cyrillic']);
+            flushSync();
+
+            expect(store.params.subsets).toEqual(['cyrillic']);
+        });
+
+        it('setSearch() sets the q param', () => {
+            store.setSearch('roboto');
+            flushSync();
+
+            expect(store.params.q).toBe('roboto');
+        });
+
+        it('setSearch() with empty string sets q to undefined', () => {
+            store.setSearch('roboto');
+            store.setSearch('');
+            flushSync();
+
+            expect(store.params.q).toBeUndefined();
+        });
+
+        it('setSort() updates the sort param', () => {
+            store.setSort('popularity');
+            flushSync();
+
+            expect(store.params.sort).toBe('popularity');
+        });
+    });
+
+    describe('filter change resets pagination', () => {
+        let store: UnifiedFontStore;
+
+        beforeEach(() => {
+            store = new UnifiedFontStore({ limit: 10 });
+            flushSync();
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('resets offset to 0 when a filter changes', () => {
+            store.setParams({ offset: 20 });
+            flushSync();
+
+            store.setParams({ q: 'roboto' });
+            flushSync();
+
+            expect(store.params.offset).toBe(0);
+        });
+
+        it('clears accumulated fonts when a filter changes', async () => {
+            const fonts = generateMockFonts(3);
+            mockedFetch.mockResolvedValue(makeResponse(fonts));
+            await store.refetch();
+            flushSync();
+            expect(store.fonts).toHaveLength(3);
+
+            store.setParams({ q: 'roboto' });
+            flushSync();
+
+            expect(store.fonts).toHaveLength(0);
+        });
+    });
+
+    describe('category getters', () => {
+        let store: UnifiedFontStore;
+
+        beforeEach(() => {
+            store = new UnifiedFontStore({ limit: 10 });
+        });
+
+        afterEach(() => {
+            store.destroy();
+            queryClient.clear();
+            vi.resetAllMocks();
+        });
+
+        it('sansSerifFonts returns only sans-serif fonts', async () => {
+            const fonts = generateMixedCategoryFonts(2);
+            mockedFetch.mockResolvedValue(makeResponse(fonts, { total: fonts.length, limit: fonts.length }));
+            await store.refetch();
+
+            expect(store.fonts).toHaveLength(10);
+            expect(store.sansSerifFonts).toHaveLength(2);
+            expect(store.sansSerifFonts.every(f => f.category === 'sans-serif')).toBe(true);
+        });
+
+        it('serifFonts returns only serif fonts', async () => {
+            const fonts = generateMixedCategoryFonts(2);
+            mockedFetch.mockResolvedValue(makeResponse(fonts, { total: fonts.length, limit: fonts.length }));
+            await store.refetch();
+
+            expect(store.serifFonts).toHaveLength(2);
+            expect(store.serifFonts.every(f => f.category === 'serif')).toBe(true);
+        });
+
+        it('displayFonts returns only display fonts', async () => {
+            const fonts = generateMixedCategoryFonts(2);
+            mockedFetch.mockResolvedValue(makeResponse(fonts, { total: fonts.length, limit: fonts.length }));
+            await store.refetch();
+
+            expect(store.displayFonts).toHaveLength(2);
+            expect(store.displayFonts.every(f => f.category === 'display')).toBe(true);
+        });
+
+        it('handwritingFonts returns only handwriting fonts', async () => {
+            const fonts = generateMixedCategoryFonts(2);
+            mockedFetch.mockResolvedValue(makeResponse(fonts, { total: fonts.length, limit: fonts.length }));
+            await store.refetch();
+
+            expect(store.handwritingFonts).toHaveLength(2);
+            expect(store.handwritingFonts.every(f => f.category === 'handwriting')).toBe(true);
+        });
+
+        it('monospaceFonts returns only monospace fonts', async () => {
+            const fonts = generateMixedCategoryFonts(2);
+            mockedFetch.mockResolvedValue(makeResponse(fonts, { total: fonts.length, limit: fonts.length }));
+            await store.refetch();
+
+            expect(store.monospaceFonts).toHaveLength(2);
+            expect(store.monospaceFonts.every(f => f.category === 'monospace')).toBe(true);
+        });
+    });
+
+    describe('destroy', () => {
+        it('calls parent destroy and filterCleanup', () => {
+            const store = new UnifiedFontStore({ limit: 10 });
+            const parentDestroySpy = vi.spyOn(Object.getPrototypeOf(Object.getPrototypeOf(store)), 'destroy');
+
+            store.destroy();
+
+            expect(parentDestroySpy).toHaveBeenCalled();
+        });
+
+        it('can be called multiple times without throwing', () => {
+            const store = new UnifiedFontStore({ limit: 10 });
+            store.destroy();
+
+            expect(() => {
+                store.destroy();
+            }).not.toThrow();
+        });
+    });
+});
--- a/src/entities/Font/model/store/unifiedFontStore/unifiedFontStore.svelte.ts
+++ b/src/entities/Font/model/store/unifiedFontStore/unifiedFontStore.svelte.ts
@@ -13,10 +13,14 @@
 */

 import type { QueryObserverOptions } from '@tanstack/query-core';
-import type { ProxyFontsParams } from '../../api';
-import { fetchProxyFonts } from '../../api';
-import type { UnifiedFont } from '../types';
-import { BaseFontStore } from './baseFontStore.svelte';
+import type { ProxyFontsParams } from '../../../api';
+import { fetchProxyFonts } from '../../../api';
+import {
+    FontNetworkError,
+    FontResponseError,
+} from '../../../lib/errors/errors';
+import type { UnifiedFont } from '../../types';
+import { BaseFontStore } from '../baseFontStore/baseFontStore.svelte';

 /**
 * Unified font store wrapping TanStack Query with Svelte 5 runes
@@ -24,7 +28,7 @@ import { BaseFontStore } from './baseFontStore.svelte';
 * Extends BaseFontStore to provide:
 * - Reactive state management
 * - TanStack Query integration for caching
- * - Dynamic parameter binding for filters
+ * - Filter change tracking with pagination reset
 * - Pagination support
 *
 * @example
@@ -93,7 +97,7 @@ export class UnifiedFontStore extends BaseFontStore<ProxyFontsParams> {
    /**
     * Track previous filter params to detect changes and reset pagination
     */
-    #previousFilterParams = $state<string>('');
+    #previousFilterParams = $state<string | null>(null);

    /**
     * Cleanup function for the filter tracking effect
@@ -130,11 +134,12 @@ export class UnifiedFontStore extends BaseFontStore<ProxyFontsParams> {
            // Effect: Sync state from Query result (Handles Cache Hits)
            $effect(() => {
                const data = this.result.data;
-                const offset = this.params.offset || 0;
+                const offset = this.params.offset ?? 0;

                // When we have data and we are at the start (offset 0),
                // we must ensure accumulatedFonts matches the fresh (or cached) data.
                // This fixes the issue where cache hits skip fetchFn side-effects.
+                // Only sync at offset 0 to avoid clearing fonts during cache hits at other offsets.
                if (offset === 0 && data && data.length > 0) {
                    this.#accumulatedFonts = data;
                }
@@ -188,37 +193,35 @@ export class UnifiedFontStore extends BaseFontStore<ProxyFontsParams> {
     * Returns the full response including pagination metadata
     */
    protected async fetchFn(params: ProxyFontsParams): Promise<UnifiedFont[]> {
-        const response = await fetchProxyFonts(params);
+        let response: Awaited<ReturnType<typeof fetchProxyFonts>>;
+        try {
+            response = await fetchProxyFonts(params);
+        } catch (cause) {
+            throw new FontNetworkError(cause);
+        }

-        // Validate response structure
        if (!response) {
-            console.error('[UnifiedFontStore] fetchProxyFonts returned undefined', { params });
-            throw new Error('Proxy API returned undefined response');
+            throw new FontResponseError('response', response);
        }
-
        if (!response.fonts) {
-            console.error('[UnifiedFontStore] response.fonts is undefined', { response });
-            throw new Error('Proxy API response missing fonts array');
+            throw new FontResponseError('response.fonts', response.fonts);
        }
-
        if (!Array.isArray(response.fonts)) {
-            console.error('[UnifiedFontStore] response.fonts is not an array', {
-                fonts: response.fonts,
-            });
-            throw new Error('Proxy API fonts is not an array');
+            throw new FontResponseError('response.fonts', response.fonts);
        }

-        // Store pagination metadata separately for derived values
        this.#paginationMetadata = {
            total: response.total ?? 0,
            limit: response.limit ?? this.params.limit ?? 50,
            offset: response.offset ?? this.params.offset ?? 0,
        };

-        // Accumulate fonts for infinite scroll
-        // Note: For offset === 0, we rely on the $effect above to handle the reset/init
-        // This prevents race conditions and double-setting.
-        if (params.offset !== 0) {
+        const offset = params.offset ?? 0;
+        if (offset === 0) {
+            // Replace accumulated fonts on offset-0 fetch
+            this.#accumulatedFonts = response.fonts;
+        } else {
+            // Append fonts when fetching at offset > 0
            this.#accumulatedFonts = [...this.#accumulatedFonts, ...response.fonts];
        }

@@ -260,6 +263,57 @@ export class UnifiedFontStore extends BaseFontStore<ProxyFontsParams> {
        return !this.isLoading && this.fonts.length === 0;
    }

+    /**
+     * Check if filter params changed and reset if needed
+     * Manually called in setParams to handle test contexts where $effect doesn't run
+     */
+    #checkAndResetFilters(newParams: Partial<ProxyFontsParams>) {
+        // Only check filter-related params (not offset/limit/page)
+        const isFilterChange = 'q' in newParams || 'providers' in newParams || 'categories' in newParams
+            || 'subsets' in newParams;
+
+        if (!isFilterChange) {
+            return;
+        }
+
+        const filterParams = JSON.stringify({
+            providers: this.params.providers,
+            categories: this.params.categories,
+            subsets: this.params.subsets,
+            q: this.params.q,
+        });
+
+        if (filterParams !== this.#previousFilterParams) {
+            // Reset offset if filter params changed
+            if (this.params.offset !== 0) {
+                // Update internal params directly to avoid recursion
+                this.updateInternalParams({ offset: 0 });
+            }
+
+            // Clear fonts if there are accumulated fonts
+            // (to avoid clearing on initial setup when no fonts exist)
+            if (this.#accumulatedFonts.length > 0) {
+                this.#accumulatedFonts = [];
+                // Clear the result to prevent effect from using stale cached data
+                this.result.data = undefined;
+            }
+
+            this.invalidate();
+            this.#previousFilterParams = filterParams;
+        }
+    }
+
+    /**
+     * Override setParams to check for filter changes
+     * @param newParams - Partial params to merge with existing
+     */
+    setParams(newParams: Partial<ProxyFontsParams>) {
+        // First update params normally
+        super.setParams(newParams);
+        // Then check if filters changed (for test contexts)
+        this.#checkAndResetFilters(newParams);
+    }
+
    /**
     * Set providers filter
     */
--- a/src/entities/Font/model/types/index.ts
+++ b/src/entities/Font/model/types/index.ts
@@ -56,3 +56,5 @@ export type {
    FontCollectionSort,
    FontCollectionState,
 } from './store';
+
+export * from './store/appliedFonts';
--- a/src/entities/Font/model/types/store/appliedFonts.ts
+++ b/src/entities/Font/model/types/store/appliedFonts.ts
@@ -0,0 +1,30 @@
+/**
+ * Configuration for a font load request.
+ */
+export interface FontLoadRequestConfig {
+    /**
+     * Unique identifier for the font (e.g., "lato", "roboto").
+     */
+    id: string;
+    /**
+     * Actual font family name recognized by the browser (e.g., "Lato", "Roboto").
+     */
+    name: string;
+    /**
+     * URL pointing to the font file (typically .ttf or .woff2).
+     */
+    url: string;
+    /**
+     * Numeric weight (100-900). Variable fonts load once per ID regardless of weight.
+     */
+    weight: number;
+    /**
+     * Variable fonts load once per ID; static fonts load per weight.
+     */
+    isVariable?: boolean;
+}
+
+/**
+ * Loading state of a font.
+ */
+export type FontLoadStatus = 'loading' | 'loaded' | 'error';
--- a/src/entities/Font/ui/FontVirtualList/FontVirtualList.svelte
+++ b/src/entities/Font/ui/FontVirtualList/FontVirtualList.svelte
@@ -15,7 +15,7 @@ import type {
 import { fade } from 'svelte/transition';
 import { getFontUrl } from '../../lib';
 import {
-    type FontConfigRequest,
+    type FontLoadRequestConfig,
    type UnifiedFont,
    appliedFontsManager,
    unifiedFontStore,
@@ -54,7 +54,7 @@ const isLoading = $derived(
 );

 function handleInternalVisibleChange(visibleItems: UnifiedFont[]) {
-    const configs: FontConfigRequest[] = [];
+    const configs: FontLoadRequestConfig[] = [];

    visibleItems.forEach(item => {
        const url = getFontUrl(item, weight);
--- a/src/shared/lib/utils/buildQueryString/buildQueryString.ts
+++ b/src/shared/lib/utils/buildQueryString/buildQueryString.ts
@@ -7,7 +7,7 @@
 * @example
 * ```ts
 * buildQueryString({ category: 'serif', subsets: ['latin', 'latin-ext'] })
- * // Returns: "?category=serif&subsets=latin%2Clatin-ext"
+ * // Returns: "?category=serif&subsets=latin&subsets=latin-ext"
 *
 * buildQueryString({ limit: 50, page: 1 })
 * // Returns: "?limit=50&page=1"
@@ -16,7 +16,7 @@
 * // Returns: ""
 *
 * buildQueryString({ search: 'hello world', active: true })
- * // Returns: "?search=hello%20world&active=true"
+ * // Returns: "?search=hello+world&active=true"
 * ```
 */

@@ -35,7 +35,7 @@ export type QueryParams = Record<string, QueryParamValue | undefined | null>;
 *
 * Handles:
 * - Primitive values (string, number, boolean) - converted to strings
- * - Arrays - comma-separated values
+ * - Arrays - multiple parameters with same key (e.g., ?key=1&key=2&key=3)
 * - null/undefined - omitted from output
 * - Special characters - URL encoded
 *
@@ -51,14 +51,12 @@ export function buildQueryString(params: QueryParams): string {
            continue;
        }

-        // Handle arrays (comma-separated values)
+        // Handle arrays - append each item as separate parameter with same key
        if (Array.isArray(value)) {
-            const joined = value
-                .filter(item => item !== undefined && item !== null)
-                .map(String)
-                .join(',');
-            if (joined) {
-                searchParams.append(key, joined);
+            for (const item of value) {
+                if (item !== undefined && item !== null) {
+                    searchParams.append(key, String(item));
+                }
            }
        } else {
            // Handle primitives
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -49,7 +49,7 @@ export default defineConfig({
            },
        },
        setupFiles: ['./vitest.setup.unit.ts'],
-        globals: false,
+        globals: true,
    },

    resolve: {