feat(editor): preserve Markdown source on copy/cut

ProseMirror's default clipboardTextSerializer uses Slice.textBetween,
which flattens every node to its inner text. Copying `## 你好` from the
editor only put `你好` on the clipboard's text/plain channel, so pasting
into VS Code, terminals, or messaging apps lost all Markdown markers.

Add a markdown-copy extension symmetric to the existing markdown-paste:
on copy/cut/drag, route the selected Slice through editor.markdown.serialize
to write the Markdown source. The text/html channel is left at ProseMirror's
default so pasting back into another ProseMirror editor still preserves
exact node structure via data-pm-slice.

Registered for both editable and readonly modes — users frequently copy
from rendered comments/issue descriptions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/views/editor/extensions/index.ts

@@ -39,6 +39,7 @@ import { BaseMentionExtension } from "./mention-extension";
 import { createMentionSuggestion } from "./mention-suggestion";
 import { CodeBlockView } from "./code-block-view";
 import { createMarkdownPasteExtension } from "./markdown-paste";
+import { createMarkdownCopyExtension } from "./markdown-copy";
 import { createSubmitExtension } from "./submit-shortcut";
 import { createBlurShortcutExtension } from "./blur-shortcut";
 import { createFileUploadExtension } from "./file-upload";
@@ -129,6 +130,10 @@ export function createEditorExtensions(
     InlineMathExtension,
     // 3-space indent so nested ordered lists survive CommonMark in ReadonlyContent.
     Markdown.configure({ indentation: { style: "space", size: 3 } }),
+    // Make Cmd+C / Cmd+X / drag write Markdown source to clipboard text/plain.
+    // Registered for both editable and readonly so users can copy from rendered
+    // comments and paste the original Markdown elsewhere.
+    createMarkdownCopyExtension(),
     FileCardExtension,
     ...(options.disableMentions
       ? []

packages/views/editor/extensions/markdown-copy.ts

@@ -0,0 +1,62 @@
+/**
+ * Markdown copy extension — make the clipboard's text/plain channel carry
+ * Markdown source instead of plain textContent.
+ *
+ * Symmetric to markdown-paste.ts:
+ *   paste:  text/plain  →  editor.markdown.parse  →  doc
+ *   copy:   slice       →  editor.markdown.serialize  →  text/plain
+ *
+ * Why: ProseMirror's default clipboardTextSerializer calls Slice.textBetween,
+ * which flattens every node to its inner text. Headings, lists, code blocks,
+ * mentions, file cards — all lose their Markdown markers. Pasting into VS
+ * Code, terminals, or messaging apps then sees only naked text.
+ *
+ * The text/html channel is left at ProseMirror's default so pasting back
+ * into another ProseMirror editor still preserves exact node structure via
+ * data-pm-slice.
+ */
+import { Extension } from "@tiptap/core";
+import { Plugin, PluginKey } from "@tiptap/pm/state";
+import type { Slice } from "@tiptap/pm/model";
+
+// Blob URLs (blob:http://…) are process-local; never let them leave the page.
+const BLOB_IMAGE_RE = /!\[[^\]]*\]\(blob:[^)]*\)\n?/g;
+
+export function createMarkdownCopyExtension() {
+  return Extension.create({
+    name: "markdownCopy",
+    addProseMirrorPlugins() {
+      const { editor } = this;
+
+      const fallback = (slice: Slice) =>
+        slice.content.textBetween(0, slice.content.size, "\n\n");
+
+      return [
+        new Plugin({
+          key: new PluginKey("markdownCopy"),
+          props: {
+            clipboardTextSerializer(slice: Slice) {
+              if (!editor.markdown) return fallback(slice);
+              try {
+                // Wrap slice content in a temp doc so the serializer walks
+                // it like a real document. Inline-only slices auto-wrap
+                // into doc → paragraph; block slices pass through.
+                const doc = editor.schema.topNodeType.create(
+                  null,
+                  slice.content,
+                );
+                const md = editor.markdown.serialize(doc.toJSON());
+                return md.replace(BLOB_IMAGE_RE, "").replace(/\n+$/, "");
+              } catch {
+                // Special selections (e.g. table cellSelection) may fail
+                // schema validation when wrapped in a doc node. Fall back
+                // so copy never breaks.
+                return fallback(slice);
+              }
+            },
+          },
+        }),
+      ];
+    },
+  });
+}