fix(viewport): 只扫描 style 块，避免大 base64 HTML 卡死

与 html2pptx 同步：parseFixedSlideHostSize 不再对整份 HTML 跑 CSS 规则正则，并补充大 data-URI 回归测试。

Author: zhaoxiongfei

src/utils/viewport.ts

@@ -151,26 +151,58 @@ export function parseDeckCssVariables(cssOrHtml: string): ViewportSize | null {
 }
 
 /**
- * Read fixed `width`/`height` px on common slide host / root selectors.
+ * Extract text content of `<style>` blocks from HTML.
+ * Used so CSS rule scanners never run against markup / data-URI blobs
+ * (those can be multi-MB lines without `{`/`}`, which makes `/[^{}@]+\{/` O(n²)).
  */
-export function parseFixedSlideHostSize(cssOrHtml: string): ViewportSize | null {
-  // Match CSS rule blocks; keep it simple (no nested @rules required for our cases).
-  const ruleRe = /([^{}@]+)\{([^{}]+)\}/g;
+export function extractInlineStyleBlocks(html: string): string {
+  const chunks: string[] = [];
+  const styleRe = /<style\b[^>]*>([\s\S]*?)<\/style>/gi;
   let match: RegExpExecArray | null;
-  while ((match = ruleRe.exec(cssOrHtml)) !== null) {
-    const selectors = match[1]!;
-    const body = match[2]!;
-    if (!SLIDE_HOST_SELECTOR_RE.test(selectors)) continue;
+  while ((match = styleRe.exec(html)) !== null) {
+    const css = match[1];
+    if (css) chunks.push(css);
+  }
+  return chunks.join('\n');
+}
 
-    const widthMatch = body.match(/(?:^|[;\s])width\s*:\s*([^;}\n]+)/i);
-    const heightMatch = body.match(/(?:^|[;\s])height\s*:\s*([^;}\n]+)/i);
-    if (!widthMatch || !heightMatch) continue;
-    const width = parseCssPxLength(widthMatch[1]!);
-    const height = parseCssPxLength(heightMatch[1]!);
-    if (width == null || height == null) continue;
-    // Ignore tiny decorative boxes accidentally matching a host selector.
-    if (width < 320 || height < 180) continue;
-    return { width, height };
+/**
+ * CSS chunks safe to scan with the CSS-rule regex.
+ * Prefer `<style>` contents when present; skip HTML markup (no style tags);
+ * otherwise treat the whole string as CSS (linked stylesheets / pure CSS).
+ */
+function cssChunksForRuleScan(cssOrHtml: string): string[] {
+  const inline = extractInlineStyleBlocks(cssOrHtml);
+  if (inline) return [inline];
+  // HTML without <style> — do not scan body / base64 blobs.
+  if (/<[a-zA-Z!/?]/.test(cssOrHtml)) return [];
+  return [cssOrHtml];
+}
+
+/**
+ * Read fixed `width`/`height` px on common slide host / root selectors.
+ * Scans only CSS (inline `<style>` blocks or pure stylesheet text), never raw HTML.
+ */
+export function parseFixedSlideHostSize(cssOrHtml: string): ViewportSize | null {
+  for (const css of cssChunksForRuleScan(cssOrHtml)) {
+    // Match CSS rule blocks; keep it simple (no nested @rules required for our cases).
+    const ruleRe = /([^{}@]+)\{([^{}]+)\}/g;
+    let match: RegExpExecArray | null;
+    while ((match = ruleRe.exec(css)) !== null) {
+      const selectors = match[1]!;
+      const body = match[2]!;
+      if (!SLIDE_HOST_SELECTOR_RE.test(selectors)) continue;
+
+      const widthMatch = body.match(/(?:^|[;\s])width\s*:\s*([^;}\n]+)/i);
+      const heightMatch = body.match(/(?:^|[;\s])height\s*:\s*([^;}\n]+)/i);
+      if (!widthMatch || !heightMatch) continue;
+      const width = parseCssPxLength(widthMatch[1]!);
+      const height = parseCssPxLength(heightMatch[1]!);
+      if (width == null || height == null) continue;
+      // Ignore tiny decorative boxes accidentally matching a host selector.
+      if (width < 320 || height < 180) continue;
+      return { width, height };
+    }
   }
   return null;
 }
@@ -241,17 +273,19 @@ function detectFromCssSources(...sources: string[]): ViewportSize | null {
  * Detect declared slide size from HTML source.
  * Order: deck-size meta → CSS size vars → viewport meta → stage JSON → fixed host px.
  * When `baseDir` is set, also reads local linked stylesheets for CSS signals.
+ * CSS detectors only see `<style>` contents + linked stylesheets (not HTML body / data URIs).
  */
 export function detectViewportFromHtml(
   html: string,
   options: { baseDir?: string } = {}
 ): ViewportSize | null {
   const linkedCss =
     options.baseDir != null ? readLocalStylesheets(html, options.baseDir) : '';
+  const inlineCss = extractInlineStyleBlocks(html);
 
   return (
     parseDeckSizeMeta(html) ??
-    detectFromCssSources(html, linkedCss) ??
+    detectFromCssSources(inlineCss, linkedCss) ??
     parseViewportMeta(html) ??
     parseStageJson(html)
   );

test/unit/viewport-detect.test.ts

@@ -108,6 +108,21 @@ describe('parseFixedSlideHostSize', () => {
     });
   });
 
+  it('reads host size from <style> inside HTML', () => {
+    const html = `<!doctype html><style>.slide-container { width: 1280px; height: 720px; }</style><img src="data:image/png;base64,${'A'.repeat(200_000)}">`;
+    assert.deepEqual(parseFixedSlideHostSize(html), {
+      width: 1280,
+      height: 720,
+    });
+  });
+
+  it('does not hang on large data-URI HTML without style host rules', () => {
+    const html = `<!doctype html><body><img src="data:image/png;base64,${'A'.repeat(500_000)}"></body>`;
+    const t0 = Date.now();
+    assert.equal(parseFixedSlideHostSize(html), null);
+    assert.ok(Date.now() - t0 < 1000, 'must finish quickly on large base64 blobs');
+  });
+
   it('ignores tiny decorative boxes', () => {
     const css = `.slide { width: 10px; height: 10px; }`;
     assert.equal(parseFixedSlideHostSize(css), null);