test(document): behavioral CCITT/DCT render fixtures prove the wasm path

Render committed synthetic fixtures through PdfViewer with the REAL
pdf.js loader and assert the canvas is non-blank (sampled dark-pixel
count). The CCITT (G4 fax) fixture exercises the shared jbig2.wasm
decode path — the same module pdf.js uses for JBIG2 — so it transitively
covers the JBIG2 acceptance criterion (the archive sample found zero
true JBIG2 docs and jbig2enc is unavailable to synthesize one). The
JPEG/DCTDecode fixture guards against regressing the natively-decoded
path. Verified the CCITT case goes red when wasmUrl is removed.

Fixtures are hermetic, committed assets (~2-5 KB each), generated with
ImageMagick — never fetched from staging at test time. CI browser mode.

Refs #708

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/document/viewer/PdfViewerFixtures.svelte.test.ts

@@ -0,0 +1,52 @@
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import PdfViewer from './PdfViewer.svelte';
+import ccittUrl from './fixtures/ccitt-g4.pdf?url';
+import jpegUrl from './fixtures/jpeg-dct.pdf?url';
+
+// Behavioral, real-render coverage of the wasm decode path. Unlike the rest of
+// the viewer tests, these use the REAL pdf.js loader (no libLoader prop) so the
+// page is actually decoded and painted, and the wasm is fetched from
+// /pdfjs-wasm/ exactly as in production. CI runs this in a real Chromium.
+// See issue #708.
+
+afterEach(cleanup);
+
+// A blank page is a uniform white canvas. A rendered page has dark glyph pixels.
+function countNonBackgroundPixels(canvas: HTMLCanvasElement): number {
+	const ctx = canvas.getContext('2d');
+	if (!ctx || canvas.width === 0 || canvas.height === 0) return 0;
+	const { data } = ctx.getImageData(0, 0, canvas.width, canvas.height);
+	let count = 0;
+	for (let i = 0; i < data.length; i += 4) {
+		const r = data[i];
+		const g = data[i + 1];
+		const b = data[i + 2];
+		const a = data[i + 3];
+		if (a > 0 && (r < 250 || g < 250 || b < 250)) count++;
+	}
+	return count;
+}
+
+async function expectNonBlankRender(url: string): Promise<void> {
+	render(PdfViewer, { url, documentId: 'fixture' });
+	await vi.waitFor(
+		() => {
+			const canvas = document.querySelector('canvas');
+			expect(canvas).not.toBeNull();
+			expect((canvas as HTMLCanvasElement).width).toBeGreaterThan(0);
+			expect(countNonBackgroundPixels(canvas as HTMLCanvasElement)).toBeGreaterThan(50);
+		},
+		{ timeout: 20000, interval: 250 }
+	);
+}
+
+describe('PdfViewer — real codec fixtures (wasm decode path)', () => {
+	it('renders a CCITT (G4 fax) scan as a non-blank page — same jbig2.wasm path JBIG2 uses', async () => {
+		await expectNonBlankRender(ccittUrl);
+	});
+
+	it('renders a DCTDecode (JPEG) PDF as a non-blank page — no regression', async () => {
+		await expectNonBlankRender(jpegUrl);
+	});
+});