fix(native-chat): keep definition-shaped user lines out of Markdown's mouth

A preserved literal marker can still vanish one layer later: CommonMark reads
`[Image #1]: /tmp/a.png` as a link reference definition, so a user turn that is
only that line renders as an empty bubble. Escape the line-leading bracket on
user turns before they reach the Markdown renderer.

Assistant prose is untouched, and the escape is output-neutral everywhere else,
including inside fenced code.

src/renderer/src/components/native-chat/NativeChatMessageList.tsx

@@ -17,6 +17,7 @@ import { stripNoiseMessages } from './native-chat-noise'
 import { foldToolMessages, splitNativeChatBlocks } from './native-chat-tool-fold'
 import { isNearBottom, shouldShowJumpToLatest, type ScrollGeometry } from './native-chat-autoscroll'
 import { isNativeChatPastedImagePath } from './native-chat-image-paste'
+import { escapeNativeChatUserMarkdown } from './native-chat-user-markdown'
 import { NativeChatToolRun } from './NativeChatToolRun'
 import { NativeChatCopyButton } from './NativeChatCopyButton'
 import { NATIVE_CHAT_STREAMING_ID } from '../../../../shared/native-chat-streaming'
@@ -174,7 +175,7 @@ function MessageRow({
             <>
               <ImageAttachmentRefs blocks={prose} />
               <CommentMarkdown
-                content={markdown}
+                content={escapeNativeChatUserMarkdown(markdown)}
                 variant="document"
                 className="text-sm"
                 onLinkClick={onLinkClick}

src/renderer/src/components/native-chat/native-chat-user-markdown.test.tsx

@@ -0,0 +1,54 @@
+import { renderToStaticMarkup } from 'react-dom/server'
+import { describe, expect, it } from 'vitest'
+import CommentMarkdown from '@/components/sidebar/CommentMarkdown'
+import { escapeNativeChatUserMarkdown } from './native-chat-user-markdown'
+
+function renderedText(content: string): string {
+  return renderToStaticMarkup(<CommentMarkdown variant="document" content={content} />)
+    .replace(/<[^>]*>/g, '')
+    .trim()
+}
+
+describe('escapeNativeChatUserMarkdown', () => {
+  // The swallow this guards against: without the escape the marker line is a
+  // CommonMark link reference definition and the whole turn renders empty.
+  it('keeps a marker line the raw Markdown pipeline drops entirely', () => {
+    const typed = '[Image #1]: /tmp/a.png'
+
+    expect(renderedText(typed)).toBe('')
+    expect(renderedText(escapeNativeChatUserMarkdown(typed))).toBe('[Image #1]: /tmp/a.png')
+  })
+
+  it('keeps a definition-shaped line that precedes other prose', () => {
+    const typed = '[Image #1]: /tmp/a.png\n\nwhat do you make of it?'
+
+    expect(renderedText(typed)).toBe('what do you make of it?')
+    expect(renderedText(escapeNativeChatUserMarkdown(typed))).toBe(
+      '[Image #1]: /tmp/a.png\nwhat do you make of it?'
+    )
+  })
+
+  it.each([
+    'keep [Image #1] literal',
+    '[Image #1] describe this',
+    'here is my note\n[Image #1]: /tmp/a.png\nend',
+    '[Image #1]: /tmp/a.png and more text after',
+    '```\n[Image #1]: /tmp/a.png\n```',
+    '- [Image #1]: /tmp/a.png',
+    'plain prose with a colon: like this'
+  ])('renders text the pipeline already showed identically', (typed) => {
+    expect(renderedText(escapeNativeChatUserMarkdown(typed))).toBe(renderedText(typed))
+  })
+
+  it.each(['keep [Image #1] literal', '```\n[Image #1]: /tmp/a.png\n```'])(
+    'leaves lines that cannot open a definition byte-identical',
+    (typed) => {
+      expect(escapeNativeChatUserMarkdown(typed)).toBe(typed)
+    }
+  )
+
+  it('escapes only the line-leading bracket, once per line', () => {
+    expect(escapeNativeChatUserMarkdown('[a]: x\n[b]: y')).toBe('\\[a]: x\n\\[b]: y')
+    expect(escapeNativeChatUserMarkdown('   [a]: x')).toBe('   \\[a]: x')
+  })
+})

src/renderer/src/components/native-chat/native-chat-user-markdown.ts

@@ -0,0 +1,25 @@
+const LINK_REFERENCE_DEFINITION_LINE = /^( {0,3})\[(?=[^\]\n]*\]:)/
+const FENCE_LINE = /^ {0,3}(?:`{3,}|~{3,})/
+
+/**
+ * A user turn is prose, not authored Markdown. CommonMark reads a line like
+ * `[Image #1]: /tmp/a.png` as a link reference definition and renders nothing at
+ * all, so a marker the transcript rules preserve would still vanish on screen.
+ * Escaping the opening bracket is output-neutral for every other line.
+ */
+export function escapeNativeChatUserMarkdown(markdown: string): string {
+  if (!markdown.includes(']:')) {
+    return markdown
+  }
+  let inFence = false
+  return markdown
+    .split('\n')
+    .map((line) => {
+      if (FENCE_LINE.test(line)) {
+        inFence = !inFence
+        return line
+      }
+      return inFence ? line : line.replace(LINK_REFERENCE_DEFINITION_LINE, '$1\\[')
+    })
+    .join('\n')
+}