fix(SliderArea): temporarily replace pretext measurements with canvas

src/widgets/ComparisonView/lib/index.ts

@@ -1,2 +1,3 @@
 export * from './utils/dotTransition';
-export * from './utils/getPretextFontString';
+export * from './utils/ensureCanvasFonts/ensureCanvasFonts';
+export * from './utils/getPretextFontString/getPretextFontString';

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

@@ -0,0 +1,301 @@
+import {
+    afterEach,
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+import { getPretextFontString } from '../getPretextFontString/getPretextFontString';
+import { ensureCanvasFonts } from './ensureCanvasFonts';
+
+const FALLBACK_FAMILY = '__glyphdiff_no_such_font_42__';
+const fallbackFont = (sizePx: number) => getPretextFontString(400, sizePx, FALLBACK_FAMILY);
+
+/**
+ * Fake Canvas2D context that returns a scripted width per font string.
+ * Tracks how many times measureText was called so tests can assert polling
+ * behavior without depending on wall-clock time.
+ */
+function createFakeCtx() {
+    const widthsByFont = new Map<string, number | (() => number)>();
+    const measureCalls: Array<{ font: string; text: string }> = [];
+    const ctx = {
+        font: '',
+        measureText(text: string) {
+            measureCalls.push({ font: ctx.font, text });
+            const entry = widthsByFont.get(ctx.font);
+            const width = typeof entry === 'function' ? entry() : entry ?? 0;
+            return { width };
+        },
+    };
+    return {
+        ctx: ctx as unknown as CanvasRenderingContext2D,
+        widthsByFont,
+        measureCalls,
+    };
+}
+
+interface MockGlobals {
+    fontsLoad: ReturnType<typeof vi.fn>;
+    rafCalls: number;
+    nowValues: number[];
+    nowIndex: { current: number };
+    restore: () => void;
+}
+
+function installGlobals(opts: {
+    /** Sequence of values returned by performance.now(); last value repeats. */
+    nowSequence: number[];
+    /** If true, OffscreenCanvas is defined and getContext returns the fake ctx. */
+    useOffscreenCanvas: boolean;
+    ctx: CanvasRenderingContext2D | null;
+}): MockGlobals {
+    const fontsLoad = vi.fn().mockResolvedValue([]);
+
+    const originals: Array<[string, PropertyDescriptor | undefined]> = [];
+    const setGlobal = (key: string, value: unknown) => {
+        originals.push([key, Object.getOwnPropertyDescriptor(globalThis, key)]);
+        Object.defineProperty(globalThis, key, {
+            configurable: true,
+            writable: true,
+            value,
+        });
+    };
+
+    setGlobal('document', {
+        fonts: { load: fontsLoad },
+        createElement: vi.fn(() => ({
+            getContext: vi.fn(() => opts.ctx),
+        })),
+    });
+
+    if (opts.useOffscreenCanvas) {
+        class FakeOffscreenCanvas {
+            getContext() {
+                return opts.ctx;
+            }
+        }
+        setGlobal('OffscreenCanvas', FakeOffscreenCanvas);
+    } else {
+        setGlobal('OffscreenCanvas', undefined);
+    }
+
+    const nowIndex = { current: 0 };
+    setGlobal('performance', {
+        now: () => opts.nowSequence[Math.min(nowIndex.current++, opts.nowSequence.length - 1)],
+    });
+
+    let rafCount = 0;
+    const rafState = { count: 0 };
+    setGlobal('requestAnimationFrame', (cb: FrameRequestCallback) => {
+        rafState.count++;
+        rafCount++;
+        Promise.resolve().then(() => cb(0));
+        return rafCount;
+    });
+
+    return {
+        fontsLoad,
+        get rafCalls() {
+            return rafState.count;
+        },
+        nowValues: opts.nowSequence,
+        nowIndex,
+        restore() {
+            for (const [key, desc] of originals) {
+                if (desc) {
+                    Object.defineProperty(globalThis, key, desc);
+                } else {
+                    delete (globalThis as any)[key];
+                }
+            }
+        },
+    } as MockGlobals;
+}
+
+describe('ensureCanvasFonts', () => {
+    let cleanup: (() => void) | undefined;
+    afterEach(() => {
+        cleanup?.();
+        cleanup = undefined;
+    });
+
+    it('awaits document.fonts.load for every font string', async () => {
+        const { ctx, widthsByFont } = createFakeCtx();
+        const fontA = getPretextFontString(400, 36, 'Roboto');
+        const fontB = getPretextFontString(400, 36, 'Smooch Sans');
+        // Real font width clearly differs from the unknown-family fallback width.
+        widthsByFont.set(fontA, 200);
+        widthsByFont.set(fontB, 130);
+        // The fallback probe uses the same size with an unknown family.
+        widthsByFont.set(fallbackFont(36), 280);
+
+        const mocks = installGlobals({
+            nowSequence: [0, 5],
+            useOffscreenCanvas: true,
+            ctx,
+        });
+        cleanup = mocks.restore;
+
+        await ensureCanvasFonts([fontA, fontB]);
+
+        expect(mocks.fontsLoad).toHaveBeenCalledTimes(2);
+        expect(mocks.fontsLoad).toHaveBeenCalledWith(fontA);
+        expect(mocks.fontsLoad).toHaveBeenCalledWith(fontB);
+    });
+
+    it('returns without polling when fonts already measure as non-fallback', async () => {
+        const { ctx, widthsByFont } = createFakeCtx();
+        const font = getPretextFontString(400, 36, 'Roboto');
+        widthsByFont.set(font, 200);
+        widthsByFont.set(fallbackFont(36), 280);
+
+        const mocks = installGlobals({
+            nowSequence: [0, 5],
+            useOffscreenCanvas: true,
+            ctx,
+        });
+        cleanup = mocks.restore;
+
+        await ensureCanvasFonts([font]);
+
+        // First iteration succeeds → no rAF needed
+        expect(mocks.rafCalls).toBe(0);
+    });
+
+    it('polls via requestAnimationFrame until measurement diverges from fallback', async () => {
+        const { ctx, widthsByFont, measureCalls } = createFakeCtx();
+        const font = getPretextFontString(400, 36, 'Roboto');
+        widthsByFont.set(fallbackFont(36), 280);
+        // Roboto reports the fallback width for the first two reads, then resolves.
+        let robotoReads = 0;
+        widthsByFont.set(font, () => {
+            robotoReads++;
+            return robotoReads <= 2 ? 280 : 200;
+        });
+
+        // Provide enough now() values for: initial fallback measurement +
+        // multiple loop iterations within the deadline.
+        const mocks = installGlobals({
+            nowSequence: [0, 10, 20, 30, 40, 50],
+            useOffscreenCanvas: true,
+            ctx,
+        });
+        cleanup = mocks.restore;
+
+        await ensureCanvasFonts([font]);
+
+        // Two iterations failed → two rAF awaits before success on the third.
+        expect(mocks.rafCalls).toBe(2);
+        // Measurement was called once for the fallback probe + three poll attempts.
+        const robotoCalls = measureCalls.filter(c => c.font === font).length;
+        expect(robotoCalls).toBe(3);
+    });
+
+    it('exits when performance.now passes the 1s deadline even if fonts never load', async () => {
+        const { ctx, widthsByFont } = createFakeCtx();
+        const font = getPretextFontString(400, 36, 'NeverLoads');
+        widthsByFont.set(fallbackFont(36), 280);
+        // Always returns fallback width → poll never finds a divergence.
+        widthsByFont.set(font, 280);
+
+        const mocks = installGlobals({
+            // Start at 0, then the next check jumps past the 1000ms deadline.
+            nowSequence: [0, 0, 1001],
+            useOffscreenCanvas: true,
+            ctx,
+        });
+        cleanup = mocks.restore;
+
+        await expect(ensureCanvasFonts([font])).resolves.toBeUndefined();
+    });
+
+    it('returns early when no canvas context is available', async () => {
+        const mocks = installGlobals({
+            nowSequence: [0],
+            useOffscreenCanvas: false,
+            ctx: null,
+        });
+        cleanup = mocks.restore;
+
+        await expect(
+            ensureCanvasFonts([getPretextFontString(400, 16, 'X')]),
+        ).resolves.toBeUndefined();
+        // fonts.load still ran; just no canvas polling.
+        expect(mocks.fontsLoad).toHaveBeenCalledTimes(1);
+        expect(mocks.rafCalls).toBe(0);
+    });
+
+    it('falls back to a DOM canvas when OffscreenCanvas is unavailable', async () => {
+        const { ctx, widthsByFont } = createFakeCtx();
+        const font = getPretextFontString(400, 36, 'Roboto');
+        widthsByFont.set(font, 200);
+        widthsByFont.set(fallbackFont(36), 280);
+
+        const mocks = installGlobals({
+            nowSequence: [0, 5],
+            useOffscreenCanvas: false,
+            ctx,
+        });
+        cleanup = mocks.restore;
+
+        await ensureCanvasFonts([font]);
+
+        expect((globalThis as any).document.createElement).toHaveBeenCalledWith('canvas');
+    });
+
+    it('uses the font size from each font string for the fallback probe', async () => {
+        const { ctx, widthsByFont, measureCalls } = createFakeCtx();
+        const fontA = getPretextFontString(400, 24, 'FontA');
+        const fontB = getPretextFontString(700, 48, 'FontB');
+        widthsByFont.set(fallbackFont(24), 150);
+        widthsByFont.set(fallbackFont(48), 360);
+        widthsByFont.set(fontA, 100);
+        widthsByFont.set(fontB, 200);
+
+        const mocks = installGlobals({
+            nowSequence: [0, 5],
+            useOffscreenCanvas: true,
+            ctx,
+        });
+        cleanup = mocks.restore;
+
+        await ensureCanvasFonts([fontA, fontB]);
+
+        const fallbackFonts = measureCalls
+            .map(c => c.font)
+            .filter(f => f.includes(FALLBACK_FAMILY));
+        expect(fallbackFonts).toContain(fallbackFont(24));
+        expect(fallbackFonts).toContain(fallbackFont(48));
+    });
+
+    it('removes a font from the pending set as soon as it diverges, leaving others to poll', async () => {
+        const { ctx, widthsByFont, measureCalls } = createFakeCtx();
+        const fontA = getPretextFontString(400, 36, 'A');
+        const fontB = getPretextFontString(400, 36, 'B');
+        widthsByFont.set(fallbackFont(36), 280);
+        // A loads immediately; B takes one extra frame.
+        widthsByFont.set(fontA, 200);
+        let bReads = 0;
+        widthsByFont.set(fontB, () => {
+            bReads++;
+            return bReads === 1 ? 280 : 150;
+        });
+
+        const mocks = installGlobals({
+            nowSequence: [0, 10, 20, 30, 40],
+            useOffscreenCanvas: true,
+            ctx,
+        });
+        cleanup = mocks.restore;
+
+        await ensureCanvasFonts([fontA, fontB]);
+
+        // A measured once (resolved iter 1). B measured twice (iter 1 fallback, iter 2 real).
+        const aCalls = measureCalls.filter(c => c.font === fontA).length;
+        const bCalls = measureCalls.filter(c => c.font === fontB).length;
+        expect(aCalls).toBe(1);
+        expect(bCalls).toBe(2);
+        expect(mocks.rafCalls).toBe(1);
+    });
+});

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

@@ -0,0 +1,62 @@
+/**
+ * Ensures a set of fonts is usable in a `<canvas>` measurement context.
+ *
+ * `document.fonts.load()` resolves once the FontFace bytes are fetched and
+ * parsed, but Chrome lazily registers fonts with the canvas measurement engine
+ * after that — `measureText` keeps returning a fallback width for some frames
+ * even though `document.fonts.check()` reports the font as loaded.
+ *
+ * Pretext caches measurements per font string forever, so a single fallback
+ * measurement during initial mount permanently poisons the cache and the
+ * comparison morph boundary drifts visibly from the thumb. This helper polls
+ * canvas measurement until each font reports a width that differs from the
+ * "unknown font family" fallback, guaranteeing the next `measureText` call
+ * sees the real glyph metrics.
+ */
+import { getPretextFontString } from '../getPretextFontString/getPretextFontString';
+
+const PROBE_TEXT = 'mmmmmmmmmm';
+const MAX_WAIT_MS = 1000;
+const DEFAULT_PROBE_SIZE_PX = 16;
+// Family unlikely to exist in any system — gives canvas's "unknown font" fallback width.
+const FALLBACK_PROBE_FAMILY = '__glyphdiff_no_such_font_42__';
+
+export async function ensureCanvasFonts(fontStrings: string[]): Promise<void> {
+    await Promise.all(fontStrings.map(f => document.fonts.load(f)));
+
+    // Pretext uses OffscreenCanvas when available; DOM canvas has separate font
+    // registration timing, so we MUST poll using the same canvas type pretext does.
+    const ctx = typeof OffscreenCanvas !== 'undefined'
+        ? new OffscreenCanvas(1, 1).getContext('2d')
+        : document.createElement('canvas').getContext('2d');
+    if (!ctx) {
+        return;
+    }
+
+    // Measure each font's "unknown font" fallback width (different per browser, per OS).
+    // Canvas uses this same fallback for any font family it can't resolve, so when the
+    // requested font finally registers, measureText will return a non-fallback width.
+    const fallbackWidths = new Map<string, number>();
+    for (const font of fontStrings) {
+        const sizeMatch = font.match(/(\d+(?:\.\d+)?)px/);
+        const sizePx = sizeMatch ? parseFloat(sizeMatch[1]) : DEFAULT_PROBE_SIZE_PX;
+        ctx.font = getPretextFontString(400, sizePx, FALLBACK_PROBE_FAMILY);
+        fallbackWidths.set(font, ctx.measureText(PROBE_TEXT).width);
+    }
+
+    const deadline = performance.now() + MAX_WAIT_MS;
+    const pending = new Set(fontStrings);
+    while (pending.size > 0 && performance.now() < deadline) {
+        for (const font of Array.from(pending)) {
+            ctx.font = font;
+            const w = ctx.measureText(PROBE_TEXT).width;
+            if (Math.abs(w - fallbackWidths.get(font)!) > 0.5) {
+                pending.delete(font);
+            }
+        }
+        if (pending.size === 0) {
+            break;
+        }
+        await new Promise<void>(resolve => requestAnimationFrame(() => resolve()));
+    }
+}

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

@@ -22,7 +22,10 @@ import { Loader } from '$shared/ui';
 import { getContext } from 'svelte';
 import { Spring } from 'svelte/motion';
 import { fade } from 'svelte/transition';
-import { getPretextFontString } from '../../lib';
+import {
+    ensureCanvasFonts,
+    getPretextFontString,
+} from '../../lib';
 import { comparisonStore } from '../../model';
 import Character from '../Character/Character.svelte';
 import Line from '../Line/Line.svelte';
@@ -144,7 +147,10 @@ $effect(() => {
     }
 });
 
-// Layout effect — depends on content, settings AND containerWidth
+// Layout effect — depends on content, settings AND containerWidth.
+// Awaits font loading into the canvas measurement context before invoking
+// the engine; otherwise pretext caches fallback-font widths globally per
+// font string, and the morph boundary drifts from the thumb visually.
 $effect(() => {
     const _text = comparisonStore.text;
     const _weight = typography.weight;
@@ -154,15 +160,22 @@ $effect(() => {
     const _width = containerWidth;
     const _isMobile = isMobile;
 
-    if (container && fontA && fontB && _width > 0) {
-        // PRETEXT API strings: "weight sizepx family"
-        const fontAStr = getPretextFontString(_weight, _size, fontA.name);
-        const fontBStr = getPretextFontString(_weight, _size, fontB.name);
+    if (!container || !fontA || !fontB || _width <= 0) {
+        return;
+    }
 
-        const padding = _isMobile ? 48 : 96;
-        const availableWidth = Math.max(0, _width - padding);
-        const lineHeight = _size * _height;
+    const fontAStr = getPretextFontString(_weight, _size, fontA.name);
+    const fontBStr = getPretextFontString(_weight, _size, fontB.name);
 
+    const padding = _isMobile ? 48 : 96;
+    const availableWidth = Math.max(0, _width - padding);
+    const lineHeight = _size * _height;
+
+    let cancelled = false;
+    ensureCanvasFonts([fontAStr, fontBStr]).then(() => {
+        if (cancelled) {
+            return;
+        }
         layoutResult = comparisonEngine.layout(
             _text,
             fontAStr,
@@ -172,7 +185,11 @@ $effect(() => {
             _spacing,
             _size,
         );
-    }
+    });
+
+    return () => {
+        cancelled = true;
+    };
 });
 
 // Dynamic backgroundSize based on isMobile — can't express this in Tailwind.