test: fix TextLayoutEngine tests — correct jsdom directive placement and canvas mock setup

fix: correct grapheme-width fallback in TextLayoutEngine for null breakableFitAdvances

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

@@ -0,0 +1,89 @@
+// @vitest-environment jsdom
+import { clearCache } from '@chenglou/pretext';
+import {
+    beforeEach,
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { installCanvasMock } from '../__mocks__/canvas';
+import { TextLayoutEngine } from './TextLayoutEngine.svelte';
+
+// Fixed-width mock: every segment is measured as (text.length * 10) px.
+// This is font-independent so we can reason about wrapping precisely.
+const CHAR_WIDTH = 10;
+
+describe('TextLayoutEngine', () => {
+    let engine: TextLayoutEngine;
+
+    beforeEach(() => {
+        // Install mock BEFORE any prepareWithSegments call.
+        // clearMeasurementCaches resets pretext's cached canvas context
+        // and segment metric caches so each test gets a clean slate.
+        installCanvasMock((_font, text) => text.length * CHAR_WIDTH);
+        clearCache();
+        engine = new TextLayoutEngine();
+    });
+
+    it('returns empty result for empty string', () => {
+        const result = engine.layout('', '400 16px "Inter"', 500, 20);
+        expect(result.lines).toHaveLength(0);
+        expect(result.totalHeight).toBe(0);
+    });
+
+    it('returns a single line when text fits within width', () => {
+        // 'ABC' = 3 chars × 10px = 30px, fits in 500px
+        const result = engine.layout('ABC', '400 16px "Inter"', 500, 20);
+        expect(result.lines).toHaveLength(1);
+        expect(result.lines[0].text).toBe('ABC');
+    });
+
+    it('breaks text into multiple lines when it exceeds width', () => {
+        // 'Hello World' — pretext will split at the space.
+        // 'Hello' = 50px, ' ' hangs, 'World' = 50px. Width = 60px forces wrap after 'Hello '.
+        const result = engine.layout('Hello World', '400 16px "Inter"', 60, 20);
+        expect(result.lines.length).toBeGreaterThan(1);
+        // First line must not exceed the container width.
+        expect(result.lines[0].width).toBeLessThanOrEqual(60);
+    });
+
+    it('assigns correct x positions to characters on a single line', () => {
+        // 'ABC': A=10px, B=10px, C=10px; all on one line in 500px container.
+        const result = engine.layout('ABC', '400 16px "Inter"', 500, 20);
+        const chars = result.lines[0].chars;
+
+        expect(chars).toHaveLength(3);
+        expect(chars[0].char).toBe('A');
+        expect(chars[0].x).toBe(0);
+        expect(chars[0].width).toBe(CHAR_WIDTH);
+
+        expect(chars[1].char).toBe('B');
+        expect(chars[1].x).toBe(CHAR_WIDTH);
+        expect(chars[1].width).toBe(CHAR_WIDTH);
+
+        expect(chars[2].char).toBe('C');
+        expect(chars[2].x).toBe(CHAR_WIDTH * 2);
+        expect(chars[2].width).toBe(CHAR_WIDTH);
+    });
+
+    it('x positions are monotonically increasing across a line', () => {
+        const result = engine.layout('ABCDE', '400 16px "Inter"', 500, 20);
+        const chars = result.lines[0].chars;
+        for (let i = 1; i < chars.length; i++) {
+            expect(chars[i].x).toBeGreaterThan(chars[i - 1].x);
+        }
+    });
+
+    it('each line has at least one char', () => {
+        const result = engine.layout('Hello World', '400 16px "Inter"', 60, 20);
+        for (const line of result.lines) {
+            expect(line.chars.length).toBeGreaterThan(0);
+        }
+    });
+
+    it('totalHeight equals lineCount * lineHeight', () => {
+        const lineHeight = 24;
+        const result = engine.layout('Hello World', '400 16px "Inter"', 60, lineHeight);
+        expect(result.totalHeight).toBe(result.lines.length * lineHeight);
+    });
+});