feat(ComparisonStore): pin fontA/fontB to prevent eviction while on-screen

src/widgets/ComparisonView/model/stores/comparisonStore.svelte.ts

@@ -134,6 +134,19 @@ export class ComparisonStore {
                     });
                 }
             });
+
+            // Effect 4: Pin fontA/fontB so eviction never removes on-screen fonts
+            $effect(() => {
+                const fa = this.#fontA;
+                const fb = this.#fontB;
+                const w = typographySettingsStore.weight;
+                if (fa) appliedFontsManager.pin(fa.id, w, fa.features?.isVariable);
+                if (fb) appliedFontsManager.pin(fb.id, w, fb.features?.isVariable);
+                return () => {
+                    if (fa) appliedFontsManager.unpin(fa.id, w, fa.features?.isVariable);
+                    if (fb) appliedFontsManager.unpin(fb.id, w, fb.features?.isVariable);
+                };
+            });
         });
     }
 

src/widgets/ComparisonView/model/stores/comparisonStore.test.ts

@@ -53,15 +53,19 @@ vi.mock('$shared/lib/helpers/createPersistentStore/createPersistentStore.svelte'
 
 // ── $entities/Font mock — keep real BatchFontStore, stub singletons ───────────
 
-vi.mock('$entities/Font', async () => {
+vi.mock('$entities/Font', async importOriginal => {
+    const actual = await importOriginal<typeof import('$entities/Font')>();
     const { BatchFontStore } = await import(
         '$entities/Font/model/store/batchFontStore.svelte'
     );
     return {
+        ...actual,
         BatchFontStore,
         fontStore: { fonts: [] },
         appliedFontsManager: {
             touch: vi.fn(),
+            pin: vi.fn(),
+            unpin: vi.fn(),
             getFontStatus: vi.fn(),
             ready: vi.fn(() => Promise.resolve()),
         },
@@ -80,9 +84,20 @@ vi.mock('$features/SetupFont', () => ({
     })),
 }));
 
+vi.mock('$features/SetupFont/model', () => ({
+    typographySettingsStore: {
+        weight: 400,
+        renderedSize: 48,
+        reset: vi.fn(),
+    },
+}));
+
 // ── Imports (after mocks) ─────────────────────────────────────────────────────
 
-import { fontStore } from '$entities/Font';
+import {
+    appliedFontsManager,
+    fontStore,
+} from '$entities/Font';
 import * as proxyFonts from '$entities/Font/api/proxy/proxyFonts';
 import { ComparisonStore } from './comparisonStore.svelte';
 
@@ -209,4 +224,55 @@ describe('ComparisonStore', () => {
             expect(store.fontB).toBeUndefined();
         });
     });
+
+    // ── Pin / Unpin ───────────────────────────────────────────────────────────
+
+    describe('Pin / Unpin (eviction guard)', () => {
+        it('pins fontA and fontB when they are loaded', async () => {
+            mockStorage._value.fontAId = mockFontA.id;
+            mockStorage._value.fontBId = mockFontB.id;
+            vi.spyOn(proxyFonts, 'fetchFontsByIds').mockResolvedValue([mockFontA, mockFontB]);
+
+            new ComparisonStore();
+
+            await vi.waitFor(() => {
+                expect(appliedFontsManager.pin).toHaveBeenCalledWith(
+                    mockFontA.id,
+                    400,
+                    mockFontA.features?.isVariable,
+                );
+                expect(appliedFontsManager.pin).toHaveBeenCalledWith(
+                    mockFontB.id,
+                    400,
+                    mockFontB.features?.isVariable,
+                );
+            }, { timeout: 2000 });
+        });
+
+        it('unpins the old font when fontA is replaced', async () => {
+            mockStorage._value.fontAId = mockFontA.id;
+            mockStorage._value.fontBId = mockFontB.id;
+            vi.spyOn(proxyFonts, 'fetchFontsByIds').mockResolvedValue([mockFontA, mockFontB]);
+
+            const store = new ComparisonStore();
+            await vi.waitFor(() => expect(store.fontA?.id).toBe(mockFontA.id), { timeout: 2000 });
+
+            const mockFontC: typeof mockFontA = { ...mockFontA, id: 'playfair', name: 'Playfair Display' };
+            vi.spyOn(proxyFonts, 'fetchFontsByIds').mockResolvedValue([mockFontC, mockFontB]);
+            store.fontA = mockFontC;
+
+            await vi.waitFor(() => {
+                expect(appliedFontsManager.unpin).toHaveBeenCalledWith(
+                    mockFontA.id,
+                    400,
+                    mockFontA.features?.isVariable,
+                );
+                expect(appliedFontsManager.pin).toHaveBeenCalledWith(
+                    mockFontC.id,
+                    400,
+                    mockFontC.features?.isVariable,
+                );
+            }, { timeout: 2000 });
+        });
+    });
 });