Fix nested Markdown code fence rendering

package.json

@@ -5,8 +5,9 @@
   "private": true,
   "type": "module",
   "scripts": {
-    "test": "npm run test:systemone && npm run test:systemone:fast && node --test test/browser-dialogs.mjs && npm run test:firefox-bidi && npm run test:runtime-lifecycle && npm run test:provider-limits && npm run test:accessibility-tree-benchmark && npm run test:toolbar-guard && npm run test:pdf-read && npm run test:pdf-selection && npm run test:social-contract && npm run test:safesocial && npm run test:build-unpacked && npm run test:attachment-drop && node test/run.js && node test/selection-scope-restoration.mjs && node scripts/benchmark-offline-relevance.mjs && npm run test:security",
+    "test": "npm run test:markdown && npm run test:systemone && npm run test:systemone:fast && node --test test/browser-dialogs.mjs && npm run test:firefox-bidi && npm run test:runtime-lifecycle && npm run test:provider-limits && npm run test:accessibility-tree-benchmark && npm run test:toolbar-guard && npm run test:pdf-read && npm run test:pdf-selection && npm run test:social-contract && npm run test:safesocial && npm run test:build-unpacked && npm run test:attachment-drop && node test/run.js && node test/selection-scope-restoration.mjs && node scripts/benchmark-offline-relevance.mjs && npm run test:security",
     "test:provider-limits": "node test/provider-model-limits.mjs",
+    "test:markdown": "node --test test/markdown-render.mjs",
     "test:accessibility-tree-benchmark": "node --test test/llm/lib/accessibility-tree-formats.test.mjs && node test/llm/accessibility-tree-benchmark.mjs --no-exact-tokenizer --check",
     "test:attachment-drop": "node test/attachment-drop.mjs",
     "test:security": "node test/security/injection-corpus.mjs",

src/chrome/src/ui/history-text.js

@@ -125,10 +125,23 @@ export function historyTextFromElement(root, { markdown = true } = {}) {
     if (markdown && tagName === 'PRE') {
       const language = String(node.parentElement?.querySelector?.('.code-lang')?.textContent || '').trim();
       ensureBreak();
-      output += `\`\`\`${language}\n`;
+      const beforeCode = output;
+      output = '';
       for (const child of Array.from(node.childNodes || [])) visit(child, false, true);
       ensureBreak();
-      output += '```';
+      const code = output;
+      // History must not reintroduce ambiguous fences around a Markdown
+      // document (or any code sample containing literal backtick runs).
+      const fenceCharacter = language.includes('`') ? '~' : '`';
+      let fenceLength = 3;
+      for (const match of code.matchAll(new RegExp(`${fenceCharacter}{3,}`, 'g'))) {
+        fenceLength = Math.max(fenceLength, match[0].length + 1);
+      }
+      const fence = fenceCharacter.repeat(fenceLength);
+      // Keep an info string separate from the fence so a label beginning with
+      // the same marker cannot extend the opener past its matching closer.
+      const info = language ? ` ${language}` : '';
+      output = `${beforeCode}${fence}${info}\n${code}${fence}`;
       return;
     }
     if (markdown && tagName === 'CODE' && !inPre) {