Merge pull request 'Feature/adaptive crossfade window' (#50) from feature/adaptive-crossfade-window into main

Reviewed-on: #50

e2e/preview-text.test.ts

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

package.json

@@ -6,8 +6,7 @@
   "type": "module",
   "sideEffects": [
     "*.css",
-    "**/router.ts",
-    "**/bindings.svelte.ts"
+    "**/router.ts"
   ],
   "scripts": {
     "dev": "vite",

src/app/providers/QueryProvider.svelte

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

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

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

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

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

src/entities/Font/domain/index.ts

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

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

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

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

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

src/entities/Font/index.ts

@@ -2,6 +2,7 @@ export {
     computeLineRenderModel,
     DualFontLayout,
     findSplitIndex,
+    windowSizeForLine,
 } from './domain';
 export type {
     ComparisonLine,
@@ -19,6 +20,7 @@ export type { FontRowSizeResolverOptions } from './lib';
 
 export {
     FontApplicator,
+    FontSampler,
     FontVirtualList,
 } from './ui';
 

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

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

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

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

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

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

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

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

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

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

src/entities/Font/testing/index.ts

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

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

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

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

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

src/entities/Font/ui/index.ts

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

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

@@ -15,10 +15,14 @@ import {
     DEFAULT_FONT_WEIGHT,
     DEFAULT_LETTER_SPACING,
     DEFAULT_LINE_HEIGHT,
-} from '$entities/Font';
+    // Deep path (not the root barrel) on purpose: pulls only these pure
+    // constants, not the entity's UI/store graph (+ @tanstack) — keeps this
+    // feature store and its spec light at import. See audit D-1.
+} from '$entities/Font/model/const/const';
 import {
     type PersistentStore,
     createPersistentStore,
+    createSingleton,
 } from '$shared/lib';
 import type { NumericControl } from '$shared/ui';
 import { SvelteMap } from 'svelte/reactivity';
@@ -166,6 +170,7 @@ export class TypographySettingsStore {
      */
     destroy(): void {
         this.#disposeEffects();
+        this.#storage.destroy();
     }
 
     /**
@@ -348,22 +353,17 @@ export function createTypographySettingsStore(
 
 export type TypographySettingsStoreInstance = ReturnType<typeof createTypographySettingsStore>;
 
-let _typographySettingsStore: TypographySettingsStoreInstance | undefined;
-
 /**
  * App-wide typography settings store, keyed for the comparison view.
  * Created on first access so its persistent-store sync effects aren't set up
  * at module load.
  */
-export function getTypographySettingsStore(): TypographySettingsStoreInstance {
-    return (_typographySettingsStore ??= createTypographySettingsStore(
-        DEFAULT_TYPOGRAPHY_CONTROLS_DATA,
-        COMPARISON_STORAGE_KEY,
-    ));
-}
+const typographySettingsStore = createSingleton(
+    () => createTypographySettingsStore(DEFAULT_TYPOGRAPHY_CONTROLS_DATA, COMPARISON_STORAGE_KEY),
+    instance => instance.destroy(),
+);
+
+export const getTypographySettingsStore = typographySettingsStore.get;
 
 // test-only reset, so specs don't share persisted typography state or leak effects
-export function __resetTypographySettingsStore() {
-    _typographySettingsStore?.destroy();
-    _typographySettingsStore = undefined;
-}
+export const __resetTypographySettingsStore = typographySettingsStore.reset;

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

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

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

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

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

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

src/features/DisplayFont/index.ts

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

src/features/DisplayFont/ui/index.ts

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

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

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

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

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

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

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

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

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

src/shared/api/queryClient.ts

@@ -27,11 +27,16 @@ export const QUERY_RETRY_BASE_DELAY_MS = 1000;
  */
 export const QUERY_RETRY_MAX_DELAY_MS = 30000;
 
+let queryClientInstance: QueryClient | undefined;
+
 /**
- * TanStack Query client instance
+ * Shared TanStack Query client (lazy singleton).
  *
- * Configured for optimal caching and refetching behavior.
- * Used by all font stores for data fetching and caching.
+ * Construction is deferred to the first call so importing this module is inert:
+ * module eval runs no `new QueryClient()`, so the module is genuinely
+ * side-effect-free and needs no `sideEffects` allowlist exception. The
+ * app-layer `QueryProvider` is the first caller; every store reuses the same
+ * instance. Matches the lazy-accessor pattern used by the font stores.
  *
  * Cache behavior:
  * - Data stays fresh for 5 minutes (staleTime)
@@ -39,30 +44,32 @@ export const QUERY_RETRY_MAX_DELAY_MS = 30000;
  * - No refetch on window focus (reduces unnecessary network requests)
  * - 3 retries with exponential backoff on failure
  */
-export const queryClient = new QueryClient({
-    defaultOptions: {
-        queries: {
-            staleTime: DEFAULT_QUERY_STALE_TIME_MS,
-            gcTime: DEFAULT_QUERY_GC_TIME_MS,
-            /**
-             * Don't refetch when window regains focus
-             */
-            refetchOnWindowFocus: false,
-            /**
-             * Refetch on mount if data is stale
-             */
-            refetchOnMount: true,
-            retry: (failureCount, error) => {
-                if (error instanceof NonRetryableError) {
-                    return false;
-                }
-                return failureCount < QUERY_RETRY_COUNT;
+export function getQueryClient(): QueryClient {
+    return (queryClientInstance ??= new QueryClient({
+        defaultOptions: {
+            queries: {
+                staleTime: DEFAULT_QUERY_STALE_TIME_MS,
+                gcTime: DEFAULT_QUERY_GC_TIME_MS,
+                /**
+                 * Don't refetch when window regains focus
+                 */
+                refetchOnWindowFocus: false,
+                /**
+                 * Refetch on mount if data is stale
+                 */
+                refetchOnMount: true,
+                retry: (failureCount, error) => {
+                    if (error instanceof NonRetryableError) {
+                        return false;
+                    }
+                    return failureCount < QUERY_RETRY_COUNT;
+                },
+                /**
+                 * Exponential backoff: 1s, 2s, 4s, 8s... capped at 30s
+                 */
+                retryDelay: attemptIndex =>
+                    Math.min(QUERY_RETRY_BASE_DELAY_MS * 2 ** attemptIndex, QUERY_RETRY_MAX_DELAY_MS),
             },
-            /**
-             * Exponential backoff: 1s, 2s, 4s, 8s... capped at 30s
-             */
-            retryDelay: attemptIndex =>
-                Math.min(QUERY_RETRY_BASE_DELAY_MS * 2 ** attemptIndex, QUERY_RETRY_MAX_DELAY_MS),
         },
-    },
-});
+    }));
+}

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

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

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

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

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

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

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

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

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

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

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

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

src/shared/lib/helpers/index.ts

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

src/shared/lib/index.ts

@@ -11,6 +11,7 @@ export {
     createPersistentStore,
     createPerspectiveManager,
     createResponsiveManager,
+    createSingleton,
     createVirtualizer,
     type Entity,
     type EntityStore,
@@ -21,6 +22,7 @@ export {
     type Property,
     type ResponsiveManager,
     responsiveManager,
+    type Singleton,
     type VirtualItem,
     type Virtualizer,
     type VirtualizerOptions,

src/shared/lib/storybook/index.ts

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

src/shared/ui/Badge/Badge.svelte

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

src/shared/ui/ComboControl/ComboControl.svelte

@@ -84,9 +84,11 @@ const displayLabel = $derived(label ?? controlLabel ?? '');
             {formattedValue()}
         </span>
     </div>
-
-    <!-- ── FULL MODE ──────────────────────────────────────────────────────────────── -->
 {:else}
+    <!--
+        FULL MODE
+        +/- buttons flanking a slider popover.
+    -->
     <div class={cn('flex items-center px-1 relative', className)}>
         <!-- Decrease button -->
         <Button

src/shared/ui/Input/index.ts

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

src/shared/ui/Label/Label.svelte

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

src/shared/ui/SearchBar/SearchBar.svelte

@@ -7,8 +7,10 @@
     They cannot pass leftIcon — it's owned by this wrapper.
 -->
 <script lang="ts">
-import { Input } from '$shared/ui/Input';
-import { inputIconSize } from '$shared/ui/Input/types';
+import {
+    Input,
+    inputIconSize,
+} from '$shared/ui/Input';
 import SearchIcon from '@lucide/svelte/icons/search';
 import type { ComponentProps } from 'svelte';
 

src/shared/ui/SidebarContainer/SidebarContainer.svelte

@@ -43,7 +43,7 @@ function close() {
 
 {#if responsive.isMobile}
     <!--
-        ── MOBILE: fixed overlay ─────────────────────────────────────────────
+        MOBILE: fixed overlay.
         Only rendered when open. Both backdrop and panel use Svelte transitions
         so they animate in and out independently.
     -->
@@ -70,7 +70,7 @@ function close() {
     {/if}
 {:else}
     <!--
-        ── DESKTOP: collapsible column ───────────────────────────────────────
+        DESKTOP: collapsible column.
         Always in the DOM — width transitions between 320px and 0.
         overflow-hidden clips the w-80 inner div during the collapse.
 

src/shared/ui/TechText/TechText.svelte

@@ -4,13 +4,13 @@
 -->
 <script lang="ts">
 import { cn } from '$shared/lib';
+import type { Snippet } from 'svelte';
 import {
     type LabelSize,
     type LabelVariant,
     labelSizeConfig,
     labelVariantConfig,
-} from '$shared/ui/Label/config';
-import type { Snippet } from 'svelte';
+} from '../labelConfig';
 
 interface Props {
     /**

src/widgets/ComparisonView/lib/index.ts

@@ -1,3 +1,4 @@
+export { computeStrutHeight } from './utils/computeStrutHeight/computeStrutHeight';
 export {
     createDotCrossfade,
     getDotTransitionParams,

src/widgets/ComparisonView/lib/utils/computeStrutHeight/computeStrutHeight.test.ts

@@ -0,0 +1,28 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { computeStrutHeight } from './computeStrutHeight';
+
+describe('computeStrutHeight', () => {
+    it('uses the centering height when the line-height is generous', () => {
+        // centering = 40/2 + 16*0.34 = 25.44; floor = 16*1.1 = 17.6 → centering wins.
+        expect(computeStrutHeight(40, 16)).toBeCloseTo(25.44, 5);
+    });
+
+    it('falls back to the ascent floor when the line-height is tight', () => {
+        // centering = 16/2 + 16*0.34 = 13.44; floor = 16*1.1 = 17.6 → floor wins.
+        expect(computeStrutHeight(16, 16)).toBeCloseTo(17.6, 5);
+    });
+
+    it('treats the floor and centering height as equal at the crossover line-height', () => {
+        // centering == floor when lineHeight = 1.52 * fontSize → 24.32 for 16px.
+        expect(computeStrutHeight(24.32, 16)).toBeCloseTo(17.6, 5);
+    });
+
+    it('scales with font size', () => {
+        // centering = 60/2 + 32*0.34 = 40.88; floor = 32*1.1 = 35.2 → centering wins.
+        expect(computeStrutHeight(60, 32)).toBeCloseTo(40.88, 5);
+    });
+});

src/widgets/ComparisonView/lib/utils/computeStrutHeight/computeStrutHeight.ts

@@ -0,0 +1,45 @@
+/**
+ * Fraction of the font size added to half the line height to drop the strut's
+ * baseline from the line box's vertical middle down to the text's optical
+ * center. Empirical: ~0.34em approximates a Latin font's midline-to-baseline
+ * offset, so glyphs sit centered rather than riding high in the line.
+ */
+const BASELINE_OFFSET_RATIO = 0.34;
+
+/**
+ * Minimum strut height as a multiple of the font size. Floors the strut above
+ * the fonts' ascent (~1em) so that at tight line-heights it stays the tallest
+ * inline box and keeps ownership of the line baseline. Empirical: 1.1 clears the
+ * tallest ascenders in the catalog's Latin fonts.
+ */
+const MIN_HEIGHT_RATIO = 1.1;
+
+/**
+ * Pixel height for a slider line's invisible baseline strut.
+ *
+ * The slider renders each line with a zero-width strut span whose box is
+ * deliberately the tallest inline box on the line. The browser pins a line box's
+ * baseline to its tallest inline box; fixing the strut's height independent of
+ * which bulk runs or window chars are currently mounted keeps the baseline (and
+ * every glyph) from jumping as the slider sweeps runs in and out. With
+ * `overflow: hidden` the strut's baseline sits at its bottom edge, so this height
+ * also sets the text's vertical position within the line box.
+ *
+ * The result is `max(centeringHeight, ascentFloor)`:
+ * - `centeringHeight = lineHeightPx / 2 + fontSizePx * BASELINE_OFFSET_RATIO`
+ *   centers the text — half the line box places the strut's bottom edge at the
+ *   vertical middle, and the offset term nudges the baseline down to the glyphs'
+ *   optical center.
+ * - `ascentFloor = fontSizePx * MIN_HEIGHT_RATIO` keeps the strut taller than the
+ *   fonts' ascent when the line-height is tight (where `centeringHeight` would
+ *   shrink below a real glyph box and let another box steal the baseline).
+ *
+ * @param lineHeightPx Line height in pixels (typography line-height × font size).
+ * @param fontSizePx   Rendered font size in pixels.
+ * @returns            Strut height in pixels.
+ */
+export function computeStrutHeight(lineHeightPx: number, fontSizePx: number): number {
+    const centeringHeight = lineHeightPx / 2 + fontSizePx * BASELINE_OFFSET_RATIO;
+    const ascentFloor = fontSizePx * MIN_HEIGHT_RATIO;
+    return Math.max(centeringHeight, ascentFloor);
+}

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

@@ -26,8 +26,11 @@ import {
 import {
     type TypographySettingsStore,
     getTypographySettingsStore,
-} from '$features/AdjustTypography/model';
-import { createPersistentStore } from '$shared/lib';
+} from '$features/AdjustTypography';
+import {
+    createPersistentStore,
+    createSingleton,
+} from '$shared/lib';
 import { untrack } from 'svelte';
 import { getPretextFontString } from '../../lib';
 
@@ -56,12 +59,6 @@ const STORAGE_KEY = 'glyphdiff:comparison';
  */
 const FONT_READY_FALLBACK_MS = 1000;
 
-// Persistent storage for selected comparison fonts
-const storage = createPersistentStore<ComparisonState>(STORAGE_KEY, {
-    fontAId: null,
-    fontBId: null,
-});
-
 /**
  * Store for managing font comparison state.
  *
@@ -100,22 +97,39 @@ export class ComparisonStore {
      * TanStack Query-backed store for efficient batch font retrieval
      */
     #fontsByIdsStore: FontsByIdsStore;
-
+    /**
+     * Paginated font catalog — source of fonts for default seeding.
+     */
     #fontCatalog: FontCatalogStore;
-
+    /**
+     * Typography settings applied to the rendered comparison.
+     */
     #typography: TypographySettingsStore;
-
+    /**
+     * Font load/cache/eviction manager; pinned to keep compared fonts resident.
+     */
     #lifecycle: FontLifecycleManager;
+    /**
+     * Per-instance persistent storage for the selected comparison fonts.
+     */
+    #storage = createPersistentStore<ComparisonState>(STORAGE_KEY, {
+        fontAId: null,
+        fontBId: null,
+    });
+    /**
+     * Disposes the constructor's $effect.root. Must be run on teardown.
+     */
+    #disposeEffects: () => void;
 
     constructor() {
         // Synchronously seed the batch store with any IDs already in storage
-        const { fontAId, fontBId } = storage.value;
+        const { fontAId, fontBId } = this.#storage.value;
         this.#fontsByIdsStore = new FontsByIdsStore(fontAId && fontBId ? [fontAId, fontBId] : []);
         this.#fontCatalog = getFontCatalog();
         this.#typography = getTypographySettingsStore();
         this.#lifecycle = getFontLifecycleManager();
 
-        $effect.root(() => {
+        this.#disposeEffects = $effect.root(() => {
             // Sync batch results → fontA / fontB
             $effect(() => {
                 const fonts = this.#fontsByIdsStore.fonts;
@@ -123,7 +137,7 @@ export class ComparisonStore {
                     return;
                 }
 
-                const { fontAId: aId, fontBId: bId } = storage.value;
+                const { fontAId: aId, fontBId: bId } = this.#storage.value;
                 if (aId) {
                     const fa = fonts.find(f => f.id === aId);
                     if (fa) {
@@ -178,7 +192,7 @@ export class ComparisonStore {
                 // 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;
+                    return this.#storage.value.fontAId !== null || this.#storage.value.fontBId !== null;
                 });
 
                 if (hasStoredSelection) {
@@ -194,7 +208,7 @@ export class ComparisonStore {
                 untrack(() => {
                     const id1 = fonts[0].id;
                     const id2 = fonts[fonts.length - 1].id;
-                    storage.value = { fontAId: id1, fontBId: id2 };
+                    this.#storage.value = { fontAId: id1, fontBId: id2 };
                     this.#fontsByIdsStore.setIds([id1, id2]);
                 });
             });
@@ -279,7 +293,7 @@ export class ComparisonStore {
      * Updates persistent storage with the current font selection.
      */
     private updateStorage() {
-        storage.value = {
+        this.#storage.value = {
             fontAId: this.#fontA?.id ?? null,
             fontBId: this.#fontB?.id ?? null,
         };
@@ -363,19 +377,28 @@ export class ComparisonStore {
         this.#fontA = undefined;
         this.#fontB = undefined;
         this.#fontsByIdsStore.setIds([]);
-        storage.clear();
+        this.#storage.clear();
         this.#typography.reset();
     }
+
+    /**
+     * Disposes reactive effects and the persistent store. Call on teardown.
+     */
+    destroy() {
+        this.#disposeEffects();
+        this.#storage.destroy();
+    }
 }
 
-let _comparisonStore: ComparisonStore | undefined;
+const comparisonStore = createSingleton(
+    () => new ComparisonStore(),
+    instance => {
+        instance.resetAll();
+        instance.destroy();
+    },
+);
 
-export function getComparisonStore(): ComparisonStore {
-    return (_comparisonStore ??= new ComparisonStore());
-}
+export const getComparisonStore = comparisonStore.get;
 
-// test-only reset, so specs don't share a live observer
-export function __resetComparisonStore() {
-    _comparisonStore?.resetAll();
-    _comparisonStore = undefined;
-}
+// test-only reset, so specs don't share a live observer or persisted state
+export const __resetComparisonStore = comparisonStore.reset;

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

@@ -11,7 +11,9 @@
 
 import type { UnifiedFont } from '$entities/Font';
 import { UNIFIED_FONTS } from '$entities/Font/testing';
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
+
+const queryClient = getQueryClient();
 import {
     beforeEach,
     describe,
@@ -44,6 +46,8 @@ const mockStorage = vi.hoisted(() => {
         configurable: true,
     });
 
+    storage.destroy = vi.fn();
+
     return storage;
 });
 
@@ -82,6 +86,12 @@ vi.mock('$entities/Font/model', async importOriginal => {
     };
 });
 
+const mockTypography = vi.hoisted(() => ({
+    weight: 400,
+    renderedSize: 48,
+    reset: vi.fn(),
+}));
+
 vi.mock('$features/AdjustTypography', () => ({
     DEFAULT_TYPOGRAPHY_CONTROLS_DATA: [],
     createTypographyControlManager: vi.fn(() => ({
@@ -89,15 +99,6 @@ vi.mock('$features/AdjustTypography', () => ({
         renderedSize: 48,
         reset: vi.fn(),
     })),
-}));
-
-const mockTypography = vi.hoisted(() => ({
-    weight: 400,
-    renderedSize: 48,
-    reset: vi.fn(),
-}));
-
-vi.mock('$features/AdjustTypography/model', () => ({
     getTypographySettingsStore: () => mockTypography,
 }));
 

src/widgets/ComparisonView/ui/Line/Line.svelte

@@ -1,7 +1,8 @@
 <!--
     Component: Line
     Renders one laid-out line as three regions: a fontA bulk run (past the
-    slider), an N-char crossfade window straddling it, and a fontB bulk run (not
+    slider), a crossfade window straddling it (its size derived per line from
+    the line's grapheme count via `windowSizeForLine`), and a fontB bulk run (not
     yet past). Bulk runs are native shaped text (kerning, ligatures); only the
     window uses per-char DOM. `split` is a primitive so the render-model
     `$derived` skips recomputation on ticks that leave it unchanged.
@@ -10,10 +11,13 @@
 import {
     type ComparisonLine,
     computeLineRenderModel,
+    windowSizeForLine,
 } from '$entities/Font';
 import { getTypographySettingsStore } from '$features/AdjustTypography';
+import { computeStrutHeight } from '../../lib';
 import { getComparisonStore } from '../../model';
 import Character from '../Character/Character.svelte';
+import SettledText from '../SettledText/SettledText.svelte';
 
 interface Props {
     /**
@@ -24,16 +28,14 @@ interface Props {
      * Count of chars the slider has passed, from `findSplitIndex`.
      */
     split: number;
-    /**
-     * Number of chars in the crossfade window around the split.
-     */
-    windowSize: number;
 }
 
-let { line, split, windowSize }: Props = $props();
+let { line, split }: Props = $props();
 
 const comparisonStore = getComparisonStore();
 
+const windowSize = $derived(windowSizeForLine(line.chars.length));
+
 const model = $derived(computeLineRenderModel(line, split, windowSize));
 
 const typography = getTypographySettingsStore();
@@ -44,36 +46,9 @@ const fontSizePx = $derived(typography.renderedSize);
 const lineHeightPx = $derived(typography.height * typography.renderedSize);
 const letterSpacingPx = $derived(typography.spacing * typography.renderedSize);
 
-/**
- * Class and style are single short bindings so the formatter keeps
- * `<span ...>{text}</span>` on one line. A wrapped text expression would leak
- * its indentation into the span content under `white-space: pre`.
- */
-const BULK_LEFT_CLASS =
-    'inline-block align-baseline leading-none text-swiss-black/75 dark:text-brand/75 transition-colors duration-300';
-const BULK_RIGHT_CLASS =
-    'inline-block align-baseline leading-none text-neutral-950 dark:text-white transition-colors duration-300';
-
-const leftStyle = $derived(`font-family:${fontA?.name ?? ''};font-size:${fontSizePx}px`);
-const rightStyle = $derived(`font-family:${fontB?.name ?? ''};font-size:${fontSizePx}px`);
-
-/**
- * Stops the whole line from jumping up or down as the slider moves. The browser
- * pins a line box's baseline to its tallest inline box, so without a fixed
- * reference the baseline (and every glyph) shifts the moment a bulk run appears
- * or disappears, or the last window char morphs to a font with a taller ascent.
- * This invisible strut is always the tallest box — `overflow: hidden` puts its
- * baseline at its bottom edge — so it owns the line baseline and holds it still.
- * Its height also sets the text's vertical position (the container is block, so
- * nothing else centers it).
- *
- * Height factors are empirical: the first term centers the text, the `* 1.1`
- * floor keeps the strut above the fonts' ascent at tight line-heights.
- */
-const strutHeightPx = $derived(Math.max(lineHeightPx / 2 + fontSizePx * 0.34, fontSizePx * 1.1));
-const strutStyle = $derived(
-    `display:inline-block;width:0;overflow:hidden;vertical-align:baseline;height:${strutHeightPx}px`,
-);
+// Invisible strut that pins the line baseline so glyphs don't jump as the
+// slider moves; `computeStrutHeight` explains the why and the formula.
+const strutHeightPx = $derived(computeStrutHeight(lineHeightPx, fontSizePx));
 </script>
 
 <!--
@@ -91,15 +66,19 @@ const strutStyle = $derived(
         style:letter-spacing="{letterSpacingPx}px"
         style:font-weight={typography.weight}
     >
-        <span style={strutStyle} aria-hidden="true"></span>
+        <span
+            class="inline-block w-0 overflow-hidden align-baseline"
+            style:height="{strutHeightPx}px"
+            aria-hidden="true"
+        ></span>
         {#if model.leftText}
-            <span class={BULK_LEFT_CLASS} style={leftStyle}>{model.leftText}</span>
+            <SettledText text={model.leftText} fontFamily={fontA.name} fontSize={fontSizePx} side="left" />
         {/if}
         {#each model.windowChars as wc (wc.key)}
             <Character char={wc.char} {fontA} {fontB} isPast={wc.isPast} fontSize={fontSizePx} />
         {/each}
         {#if model.rightText}
-            <span class={BULK_RIGHT_CLASS} style={rightStyle}>{model.rightText}</span>
+            <SettledText text={model.rightText} fontFamily={fontB.name} fontSize={fontSizePx} side="right" />
         {/if}
     </div>
 {/if}

src/widgets/ComparisonView/ui/SettledText/SettledText.svelte

@@ -0,0 +1,37 @@
+<!--
+    Component: SettledText
+    One side's text settled in a single font — left is fontA the slider has
+    passed, right is fontB not yet reached. A native shaped run (kerning,
+    ligatures); the crossfading middle uses per-char Character cells instead.
+-->
+<script lang="ts">
+interface Props {
+    /**
+     * Run text.
+     */
+    text: string;
+    /**
+     * CSS font-family name.
+     */
+    fontFamily: string;
+    /**
+     * Font size in px.
+     */
+    fontSize: number;
+    /**
+     * Window side — selects the color treatment.
+     */
+    side: 'left' | 'right';
+}
+
+let { text, fontFamily, fontSize, side }: Props = $props();
+
+// Left (fontA, passed) is dimmed; right (fontB, pending) is full-strength.
+const SIDE_CLASS: Record<Props['side'], string> = {
+    left:
+        'inline-block align-baseline leading-none text-swiss-black/75 dark:text-brand/75 transition-colors duration-300',
+    right: 'inline-block align-baseline leading-none text-neutral-950 dark:text-white transition-colors duration-300',
+};
+</script>
+
+<span class={SIDE_CLASS[side]} style:font-family="'{fontFamily}'" style:font-size="{fontSize}px">{text}</span>

src/widgets/ComparisonView/ui/Sidebar/Sidebar.svelte

@@ -52,7 +52,6 @@ const side = $derived<Side>(comparisonStore.side);
         className,
     )}
 >
-    <!-- ── Header: title + A/B toggle ────────────────────────────────── -->
     <div
         class="
             p-6 shrink-0
@@ -96,14 +95,13 @@ const side = $derived<Side>(comparisonStore.side);
         </ButtonGroup>
     </div>
 
-    <!-- ── Main: content area (no scroll - VirtualList handles scrolling) ─────────────────────────────── -->
+    <!-- No scroll here; VirtualList handles scrolling -->
     <div class="flex-1 min-h-0 surface-canvas">
         {#if main}
             {@render main()}
         {/if}
     </div>
 
-    <!-- ── Bottom: fixed controls ─────────────────────────────────────── -->
     {#if controls}
         <div
             class="

src/widgets/ComparisonView/ui/SliderArea/SliderArea.svelte

@@ -107,12 +107,6 @@ const layout = new DualFontLayout();
 
 let layoutResult = $state<ComparisonResult>({ lines: [], totalHeight: 0 });
 
-/**
- * N-window size for the per-char crossfade zone around the slider split.
- * Tuned so chars complete their 100ms opacity crossfade before exiting the window.
- */
-const WINDOW_SIZE = 5;
-
 // Track container width changes (window resize, sidebar toggle, etc.)
 $effect(() => {
     if (!container) {
@@ -344,7 +338,7 @@ $effect(() => {
                     >
                         {#each layoutResult.lines as line, lineIdx (lineIdx)}
                             {@const split = findSplitIndex(line, sliderPos, containerWidth)}
-                            <Line {line} {split} windowSize={WINDOW_SIZE} />
+                            <Line {line} {split} />
                         {/each}
                     </div>
 

src/widgets/SampleList/model/stores/layoutStore/layoutStore.svelte.ts

@@ -16,8 +16,11 @@
  * - Desktop Large (>= 1280px): 4 columns
  */
 
-import { createPersistentStore } from '$shared/lib';
-import { responsiveManager } from '$shared/lib';
+import {
+    createPersistentStore,
+    createSingleton,
+    responsiveManager,
+} from '$shared/lib';
 
 export type LayoutMode = 'list' | 'grid';
 
@@ -144,22 +147,25 @@ class LayoutManager {
         this.#mode = DEFAULT_CONFIG.mode;
         this.#store.clear();
     }
-}
 
-let _layoutManager: LayoutManager | undefined;
+    /**
+     * Dispose the persistent store's save effect. Call on store disposal.
+     */
+    destroy(): void {
+        this.#store.destroy();
+    }
+}
 
 /**
  * App-wide layout manager, created on first access. Lazy so its persisted
  * layout preference isn't read at module load.
  */
-export function getLayoutManager(): LayoutManager {
-    return (_layoutManager ??= new LayoutManager());
-}
+const layoutManager = createSingleton(() => new LayoutManager(), instance => instance.destroy());
+
+export const getLayoutManager = layoutManager.get;
 
 // test-only reset, so specs don't share persisted layout state
-export function __resetLayoutManager() {
-    _layoutManager = undefined;
-}
+export const __resetLayoutManager = layoutManager.reset;
 
 // Export class for testing purposes
 export { LayoutManager };

src/widgets/SampleList/model/stores/layoutStore/layoutStore.test.ts

@@ -7,6 +7,7 @@ import {
     describe,
     expect,
     it,
+    vi,
 } from 'vitest';
 
 // Helper to flush Svelte effects (they run in microtasks)
@@ -69,12 +70,17 @@ describe('layoutStore', () => {
         });
 
         it('should default to list mode when localStorage has invalid data', async () => {
+            // createPersistentStore logs and swallows the parse error; silence
+            // the expected warn so it doesn't pollute test output, and assert it.
+            const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
             localStorage.setItem(STORAGE_KEY, 'invalid json');
 
             const { LayoutManager } = await import('./layoutStore.svelte');
             const manager = new LayoutManager();
 
             expect(manager.mode).toBe('list');
+            expect(warnSpy).toHaveBeenCalled();
+            warnSpy.mockRestore();
         });
 
         it('should default to list mode when localStorage has empty object', async () => {

src/widgets/SampleList/ui/SampleList/SampleList.svelte

@@ -6,6 +6,7 @@
 -->
 <script lang="ts">
 import {
+    FontSampler,
     FontVirtualList,
     createFontRowSizeResolver,
     getFontCatalog,
@@ -15,7 +16,6 @@ import {
     TypographyMenu,
     getTypographySettingsStore,
 } from '$features/AdjustTypography';
-import { FontSampler } from '$features/DisplayFont';
 import { throttle } from '$shared/lib/utils';
 import { Skeleton } from '$shared/ui';
 import { getLayoutManager } from '../../model';
@@ -127,7 +127,7 @@ const fontRowHeight = $derived.by(() =>
                 getFontStatus reads a $state SvelteMap, so the row stays reactive.
             -->
             {@const status = fontLifecycleManager.getFontStatus(font.id, typographySettingsStore.weight, font.features?.isVariable)}
-            <FontSampler bind:text {font} {index} {status} />
+            <FontSampler bind:text {font} {index} {status} typography={typographySettingsStore} />
         {/snippet}
     </FontVirtualList>
 

tsconfig.json

@@ -24,7 +24,6 @@
 
     /* Path Aliases */
     "paths": {
-      "$lib/*": ["./src/lib/*"],
       "$app/*": ["./src/app/*"],
       "$widgets/*": ["./src/widgets/*"],
       "$shared/*": ["./src/shared/*"],

vite.config.ts

@@ -6,7 +6,6 @@ export default defineConfig({
     plugins: [svelte(), tailwindcss()],
     resolve: {
         alias: {
-            $lib: '/src/lib',
             $app: '/src/app',
             $shared: '/src/shared',
             $entities: '/src/entities',

vitest.config.browser.ts

@@ -23,7 +23,6 @@ export default defineConfig({
 
     resolve: {
         alias: {
-            $lib: path.resolve(__dirname, './src/lib'),
             $app: path.resolve(__dirname, './src/app'),
             $shared: path.resolve(__dirname, './src/shared'),
             $entities: path.resolve(__dirname, './src/entities'),

vitest.config.component.ts

@@ -21,7 +21,6 @@ export default defineConfig({
     resolve: {
         conditions: ['browser'],
         alias: {
-            $lib: path.resolve(__dirname, './src/lib'),
             $app: path.resolve(__dirname, './src/app'),
             $shared: path.resolve(__dirname, './src/shared'),
             $entities: path.resolve(__dirname, './src/entities'),

vitest.config.ts

@@ -53,7 +53,6 @@ export default defineConfig({
     resolve: {
         conditions: process.env.VITEST ? ['browser'] : undefined,
         alias: {
-            $lib: path.resolve(__dirname, './src/lib'),
             $app: path.resolve(__dirname, './src/app'),
             $shared: path.resolve(__dirname, './src/shared'),
             $entities: path.resolve(__dirname, './src/entities'),

vitest.setup.component.ts

@@ -1,4 +1,4 @@
-import { queryClient } from '$shared/api/queryClient';
+import { getQueryClient } from '$shared/api/queryClient';
 import * as matchers from '@testing-library/jest-dom/matchers';
 import { cleanup } from '@testing-library/svelte';
 import {
@@ -14,7 +14,7 @@ expect.extend(matchers);
 
 afterEach(() => {
     cleanup();
-    queryClient.clear();
+    getQueryClient().clear();
 });
 
 // Mock window.matchMedia for components that use it