fix(mobile): unclosed code fences in HTML blocks no longer freeze PR overviews (#26179)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: juliusmarminge

apps/mobile/src/lib/markdownDocument.ts

@@ -163,8 +163,10 @@ function markdownHtmlTags(content: string): RegExpExecArray[] {
       }
     }
     if (closingIndex === -1) {
-      if (fenced) codeRanges.push({ start: opening.index, end: content.length });
-      continue;
+      if (!fenced) continue;
+      // An unclosed fence runs to the end of the content, so no later opener matters.
+      codeRanges.push({ start: opening.index, end: content.length });
+      break;
     }
     const closing = delimiters[closingIndex]!;
     codeRanges.push({ start: opening.index, end: closing.index + closing[0].length });

apps/mobile/src/lib/markdownHtml.test.ts

@@ -75,6 +75,16 @@ describe("HTML in native Markdown", () => {
     expect(result).toContain('`<img src="inline.png">`');
   });
 
+  it("treats an unclosed fence in an HTML block as code through the end", () => {
+    // Sized so the quadratic rescan cannot finish within the test timeout.
+    const fences = Array.from({ length: 40_000 }, () => "```x y").join("\n");
+    const markdown = `<details>\n<img src="before.png">\n${fences}\n<img src="after.png">\n</details>`;
+    const result = markdownHtmlContent(markdown);
+    expect(result).toContain("![](<before.png>)");
+    expect(result).not.toContain("<details>");
+    expect(result).toContain('<img src="after.png">\n</details>');
+  });
+
   it("links HTML anchors in a table cell without breaking the row at <br>", () => {
     const document = parseMarkdownDocument(
       "| Status | Preview URL |\n| - | - |\n| Done | <a href='https://commit.example.com'>Commit</a><br><a href='https://branch.example.com'>Branch</a> |",