perf(editor): reduce long-document decoration work and fix fold navigation

frontend/src/features/editor/codeBlockLabels.spec.ts

@@ -1,5 +1,5 @@
 // @vitest-environment happy-dom
-import { expect, it } from 'vitest'
+import { expect, it, vi } from 'vitest'
 import { installCodeBlockLabels } from './codeBlockLabels'
 
 it('keeps footer labels in sync when the language changes and stops after disposal', async () => {
@@ -16,3 +16,21 @@ it('keeps footer labels in sync when the language changes and stops after dispos
   await new Promise(resolve => setTimeout(resolve, 0))
   expect(block.dataset.languageLabel).toBe('TypeScript')
 })
+
+it('ignores code text mutations and discovers newly inserted code blocks', async () => {
+  const root = document.createElement('div')
+  root.innerHTML = '<div class="milkdown-code-block"><button class="language-button">Python</button><div class="cm-content">old</div></div>'
+  const dispose = installCodeBlockLabels(root)
+  const scan = vi.spyOn(root, 'querySelectorAll')
+  try {
+    root.querySelector('.cm-content')!.textContent = 'new code'
+    await new Promise(resolve => setTimeout(resolve, 0))
+    expect(scan).not.toHaveBeenCalled()
+    const block = document.createElement('div')
+    block.className = 'milkdown-code-block'
+    block.innerHTML = '<button class="language-button">Rust</button>'
+    root.append(block)
+    await new Promise(resolve => setTimeout(resolve, 0))
+    expect(block.dataset.languageLabel).toBe('Rust')
+  } finally { dispose(); scan.mockRestore() }
+})