fix(editor): choose safe markdown code fences

src/renderer/src/components/editor/rich-markdown-code-block-markdown.ts

@@ -0,0 +1,38 @@
+import type { JSONContent } from '@tiptap/core'
+
+type MarkdownCodeBlockRenderHelpers = {
+  renderChildren: (nodes: JSONContent[]) => string
+}
+
+function longestFenceRun(text: string, character: '`' | '~'): number {
+  let longest = 0
+  let current = 0
+  for (const value of text) {
+    if (value === character) {
+      current += 1
+      longest = Math.max(longest, current)
+    } else {
+      current = 0
+    }
+  }
+  return longest
+}
+
+function chooseFence(text: string): { character: '`' | '~'; length: number } {
+  const backtickLength = Math.max(3, longestFenceRun(text, '`') + 1)
+  const tildeLength = Math.max(3, longestFenceRun(text, '~') + 1)
+  return tildeLength < backtickLength
+    ? { character: '~', length: tildeLength }
+    : { character: '`', length: backtickLength }
+}
+
+export function renderRichMarkdownCodeBlock(
+  node: JSONContent,
+  helpers: MarkdownCodeBlockRenderHelpers
+): string {
+  const language = typeof node.attrs?.language === 'string' ? node.attrs.language : ''
+  const body = helpers.renderChildren(node.content ?? [])
+  const fence = chooseFence(body)
+  const marker = fence.character.repeat(fence.length)
+  return [`${marker}${language}`, body, marker].join('\n')
+}

src/renderer/src/components/editor/rich-markdown-extensions.ts

@@ -45,6 +45,7 @@ import { RichMarkdownInlineMath } from './rich-markdown-inline-math'
 import { RichMarkdownCodeBlockLowlight } from './rich-markdown-lowlight'
 import { RichMarkdownTaskList } from './rich-markdown-task-list'
 import { createCachedLowlight } from './rich-markdown-lowlight-cache'
+import { renderRichMarkdownCodeBlock } from './rich-markdown-code-block-markdown'
 
 const lowlight = createCachedLowlight(createLowlight(common))
 
@@ -83,6 +84,7 @@ export function createRichMarkdownExtensions({
     RichMarkdownParagraph,
     RichMarkdownCode,
     RichMarkdownCodeBlockLowlight.extend({
+      renderMarkdown: renderRichMarkdownCodeBlock,
       addNodeView() {
         // Why: RichMarkdownCodeBlock never reads getPos, so it must not re-render
         // just because earlier edits shifted this block's document position.

src/renderer/src/components/editor/rich-markdown-list-item.test.ts

@@ -60,4 +60,16 @@ describe('ordered list continuation serialization', () => {
     }
     expect(current).toBe('1. one\n   ```ts\n   const value = 1\n   ```\n2. two')
   })
+
+  it('preserves meaningful indentation inside an ordered-item fence', () => {
+    const source = '1. one\n\n   ```\n     indented\n   ```\n2. two'
+    expect(roundTrip(source)).toBe('1. one\n   ```\n     indented\n   ```\n2. two')
+  })
+
+  it('chooses a non-colliding fence for fence-shaped code content', () => {
+    const source = '~~~\n```\n~~~'
+    const canonical = roundTrip(source)
+    expect(canonical).toBe(source)
+    expect(roundTrip(canonical)).toBe(canonical)
+  })
 })