fix(editor): honor Plain text code block highlighting (#35111)

* fix(editor): honor Plain text code block highlighting

* fix(editor): trim Plain text highlighting tests and drop redundant guard

---------

Co-authored-by: OrcaWin <293788423+OrcaWin@users.noreply.github.com>

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

@@ -40,8 +40,12 @@ function highlightBlock(
   defaultLanguage: string | null | undefined
 ): Decoration[] {
   const language = (block.node.attrs.language as string | null | undefined) || defaultLanguage
+  // Why: unlabeled fences show as Plain text, matching the preview's rehype-highlight.
+  if (!language) {
+    return []
+  }
   const highlighted =
-    language && (languages.has(language) || lowlight.registered(language))
+    languages.has(language) || lowlight.registered(language)
       ? lowlight.highlight(language, block.node.textContent)
       : lowlight.highlightAuto(block.node.textContent)
   const decorations: Decoration[] = []

src/renderer/src/components/editor/rich-markdown-plain-code.test.ts

@@ -0,0 +1,49 @@
+// @vitest-environment happy-dom
+import { Editor } from '@tiptap/core'
+import StarterKit from '@tiptap/starter-kit'
+import { common, createLowlight } from 'lowlight'
+import { afterEach, expect, it } from 'vitest'
+import { RichMarkdownCodeBlockLowlight } from './rich-markdown-lowlight'
+
+const editors: Editor[] = []
+afterEach(() => editors.splice(0).forEach((editor) => editor.destroy()))
+
+function mount(language: string | null) {
+  const editor = new Editor({
+    element: document.createElement('div'),
+    extensions: [
+      StarterKit.configure({ codeBlock: false, trailingNode: false }),
+      RichMarkdownCodeBlockLowlight.configure({
+        lowlight: createLowlight(common),
+        defaultLanguage: null
+      })
+    ],
+    content: {
+      type: 'doc',
+      content: [
+        {
+          type: 'codeBlock',
+          attrs: { language },
+          content: [{ type: 'text', text: 'const answer = 42;' }]
+        }
+      ]
+    }
+  })
+  editors.push(editor)
+  const spans = () => editor.view.dom.querySelectorAll('[class*="hljs-"]').length
+  return { editor, spans }
+}
+
+it('keeps unlabeled code blocks uncolored after edits', () => {
+  const { editor, spans } = mount(null)
+  expect(spans()).toBe(0)
+  editor.view.dispatch(editor.state.tr.insertText('changed ', 1))
+  expect(spans()).toBe(0)
+})
+
+it('removes syntax colors when choosing Plain text', () => {
+  const { editor, spans } = mount('javascript')
+  expect(spans()).toBeGreaterThan(0)
+  editor.view.dispatch(editor.state.tr.setNodeMarkup(0, undefined, { language: '' }))
+  expect(spans()).toBe(0)
+})