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>
