fix(comparisonStore): preserve stored selection on cold load

The seed-defaults effect fired whenever fontA/fontB were still
undefined, including the window between constructor reading storage
and the per-id batch resolving. On a slow batch or fast catalog the
effect clobbered storage with catalog[0]/catalog[N-1], losing the
user's pick on reload.

Now bails when storage already holds IDs, and reads storage via
untrack so per-font selection writes don't re-trigger the effect.

Adds a deterministic regression test that controls catalog/batch
ordering via mockImplementation timing.

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

@@ -102,7 +102,7 @@ export class ComparisonStore {
         this.#fontsByIdsStore = new FontsByIdsStore(fontAId && fontBId ? [fontAId, fontBId] : []);
 
         $effect.root(() => {
-            // Effect 1: Sync batch results → fontA / fontB
+            // Sync batch results → fontA / fontB
             $effect(() => {
                 const fonts = this.#fontsByIdsStore.fonts;
                 if (fonts.length === 0) {
@@ -124,7 +124,7 @@ export class ComparisonStore {
                 }
             });
 
-            // Effect 2: Trigger font loading whenever selection or weight changes
+            // Trigger font loading whenever selection or weight changes
             $effect(() => {
                 const fa = this.#fontA;
                 const fb = this.#fontB;
@@ -154,24 +154,38 @@ export class ComparisonStore {
                 }
             });
 
-            // Effect 3: Set default fonts when storage is empty
+            // Set default fonts when storage is empty
             $effect(() => {
                 if (this.#fontA && this.#fontB) {
                     return;
                 }
 
+                // Don't clobber a pending rehydration - only seed when storage is empty.
+                // Untracked: only the catalog load should drive this effect, not the
+                // user's storage writes that happen as a result of normal selection.
+                const hasStoredSelection = untrack(() => {
+                    return storage.value.fontAId !== null || storage.value.fontBId !== null;
+                });
+
+                if (hasStoredSelection) {
+                    return;
+                }
+
                 const fonts = fontCatalogStore.fonts;
-                if (fonts.length >= 2) {
+
+                if (fonts.length < 2) {
+                    return;
+                }
+
                 untrack(() => {
                     const id1 = fonts[0].id;
                     const id2 = fonts[fonts.length - 1].id;
                     storage.value = { fontAId: id1, fontBId: id2 };
                     this.#fontsByIdsStore.setIds([id1, id2]);
                 });
-                }
             });
 
-            // Effect 4: Pin fontA/fontB so eviction never removes on-screen fonts
+            // Pin fontA/fontB so eviction never removes on-screen fonts
             $effect(() => {
                 const fa = this.#fontA;
                 const fb = this.#fontB;

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

@@ -165,6 +165,46 @@ describe('ComparisonStore', () => {
                 expect(mockStorage._value.fontBId).toBe(mockFontB.id);
             });
         });
+
+        /**
+         * Regression: when storage already holds the user's selection, the
+         * seed-defaults effect must bail out — even if it fires before the
+         * per-id batch returns (catalog wins the race on slow networks or
+         * cold reloads). Pre-fix the effect only checked fontA/fontB, both
+         * still undefined at this point, and clobbered storage with whatever
+         * the catalog had as fonts[0] / fonts[N-1].
+         */
+        it('should not overwrite stored IDs when batch is still in flight', async () => {
+            const seededA = UNIFIED_FONTS.lato;
+            const seededB = UNIFIED_FONTS.montserrat;
+
+            mockStorage._value.fontAId = seededA.id;
+            mockStorage._value.fontBId = seededB.id;
+
+            // Catalog defaults differ from the stored selection — if the
+            // effect mis-seeds, storage will flip to roboto / open-sans.
+            (fontCatalogStore as any).fonts = [mockFontA, mockFontB];
+
+            // Delay the batch so the catalog-driven effect runs first.
+            vi.spyOn(proxyFonts, 'fetchFontsByIds').mockImplementation(
+                () => new Promise(r => setTimeout(() => r([seededA, seededB]), 50)),
+            );
+
+            const store = new ComparisonStore();
+
+            // Let the catalog effect run; storage must be untouched.
+            await new Promise(r => setTimeout(r, 10));
+            expect(mockStorage._value.fontAId).toBe(seededA.id);
+            expect(mockStorage._value.fontBId).toBe(seededB.id);
+
+            // Batch resolves with the seeded selection — fontA/B must match.
+            await vi.waitFor(() => {
+                expect(store.fontA?.id).toBe(seededA.id);
+                expect(store.fontB?.id).toBe(seededB.id);
+            }, { timeout: 2000 });
+            expect(mockStorage._value.fontAId).toBe(seededA.id);
+            expect(mockStorage._value.fontBId).toBe(seededB.id);
+        });
     });
 
     describe('Aggregate Loading State', () => {