From cd8e98fdf9b1f3c58aa3037d2c7dd4efac528e56 Mon Sep 17 00:00:00 2001
From: Neil <4138956+nwparker@users.noreply.github.com>
Date: Sat, 12 Sep 2026 01:29:09 -0700
Subject: [PATCH] fix: prevent mobile markdown parser from stalling on
 unsupported blocks (#20313)

---
 .../mobile-markdown-parser-progress.test.ts   | 46 +++++++++++++++++++
 .../src/components/mobile-markdown-parser.ts  | 11 +++--
 2 files changed, 53 insertions(+), 4 deletions(-)
 create mode 100644 mobile/src/components/mobile-markdown-parser-progress.test.ts

diff --git a/mobile/src/components/mobile-markdown-parser-progress.test.ts b/mobile/src/components/mobile-markdown-parser-progress.test.ts
new file mode 100644
index 00000000000..e5cf3181d0c
--- /dev/null
+++ b/mobile/src/components/mobile-markdown-parser-progress.test.ts
@@ -0,0 +1,46 @@
+import { runInNewContext } from 'node:vm'
+import { describe, expect, it } from 'vitest'
+import { normalizeMobileMarkdownPreviewHtml } from './mobile-markdown-preview-html'
+import { parseMobileMarkdown } from './mobile-markdown-parser'
+
+function parseWithDeadline(input: string) {
+  // A synchronous parser loop must fail without hanging the test worker.
+  return runInNewContext('parse(input)', { parse: parseMobileMarkdown, input }, { timeout: 250 })
+}
+
+describe('mobile Markdown parser progress', () => {
+  it.each(['```c++', '```c#', '``` ts', '```ts title="file.ts"', '````', '```!'])(
+    'retains unsupported fence %s as paragraph text',
+    (fence) => {
+      expect(parseWithDeadline(fence)).toEqual([{ type: 'paragraph', text: fence }])
+      expect(parseWithDeadline(`before\n${fence}\nafter`)).toEqual([
+        { type: 'paragraph', text: `before\n${fence}\nafter` }
+      ])
+    }
+  )
+
+  it.each(['# ', '## ', '###### ', '#\t'])('consumes incomplete heading %s', (heading) => {
+    expect(parseWithDeadline(`${heading}\nnext`)).toEqual([
+      { type: 'paragraph', text: `${heading}\nnext` }
+    ])
+  })
+
+  it('handles unsupported fences through the production preview normalization', () => {
+    const input = normalizeMobileMarkdownPreviewHtml('```c++\nint x = 1;')
+    expect(parseWithDeadline(input)).toEqual([{ type: 'paragraph', text: input }])
+  })
+
+  it('recognizes a supported fence after unsupported fence text', () => {
+    expect(parseWithDeadline('```c++\n```ts\nconst x = 1\n```')).toEqual([
+      { type: 'paragraph', text: '```c++' },
+      { type: 'code', text: 'const x = 1', language: 'ts', closed: true }
+    ])
+  })
+
+  it('preserves supported fences and their streaming state', () => {
+    expect(parseWithDeadline('before\n```ts\nconst x = 1')).toEqual([
+      { type: 'paragraph', text: 'before' },
+      { type: 'code', text: 'const x = 1', language: 'ts', closed: false }
+    ])
+  })
+})
diff --git a/mobile/src/components/mobile-markdown-parser.ts b/mobile/src/components/mobile-markdown-parser.ts
index fa16b3b9310..2ee83236007 100644
--- a/mobile/src/components/mobile-markdown-parser.ts
+++ b/mobile/src/components/mobile-markdown-parser.ts
@@ -8,6 +8,9 @@ export type MobileMarkdownBlock =
   | { type: 'table'; headers: string[]; rows: string[][] }
   | { type: 'rule' }
 
+const HEADING = /^(#{1,6})\s+(.+)$/
+const CODE_FENCE = /^```([A-Za-z0-9_-]+)?\s*$/
+
 function splitTableRow(line: string): string[] {
   return line
     .trim()
@@ -34,7 +37,7 @@ export function parseMobileMarkdown(content: string): MobileMarkdownBlock[] {
       continue
     }
 
-    const fence = line.match(/^```([A-Za-z0-9_-]+)?\s*$/)
+    const fence = line.match(CODE_FENCE)
     if (fence) {
       index += 1
       const code: string[] = []
@@ -80,7 +83,7 @@ export function parseMobileMarkdown(content: string): MobileMarkdownBlock[] {
       continue
     }
 
-    const heading = line.match(/^(#{1,6})\s+(.+)$/)
+    const heading = line.match(HEADING)
     if (heading) {
       blocks.push({ type: 'heading', level: heading[1]!.length, text: heading[2]!.trim() })
       index += 1
@@ -121,8 +124,8 @@ export function parseMobileMarkdown(content: string): MobileMarkdownBlock[] {
     while (
       index < lines.length &&
       lines[index]?.trim() &&
-      !(lines[index] ?? '').startsWith('```') &&
-      !/^(#{1,6})\s+/.test(lines[index] ?? '') &&
+      !CODE_FENCE.test(lines[index] ?? '') &&
+      !HEADING.test(lines[index] ?? '') &&
       !/^>\s?/.test(lines[index] ?? '') &&
       !/^\s*(?:[-*+]|\d+[.)])\s+/.test(lines[index] ?? '') &&
       !/^\s*(-{3,}|\*{3,}|_{3,})\s*$/.test(lines[index] ?? '')