Copying a text-only selection writes its Markdown unescaped

[Azganoth]

Summary

A selection holding only text is copied as the characters themselves rather than as Markdown, so text the editor deliberately keeps literal reaches the clipboard as live source. Pasting it elsewhere produces a heading, a link, or emphasis the author never wrote. The save path escapes the same text correctly, so the file and the clipboard disagree about what the document says.

Steps to reproduce

1. Open a document containing \# not a heading.
2. Select the paragraph and copy it.
3. Paste into another Markdown editor, or anywhere the plain-text flavor is the one used.

Expected behavior

The clipboard's Markdown carries the escapes the save path writes, so text the editor keeps literal stays literal wherever it is pasted. Rendered content, the HTML flavor, and selections that already escape correctly are unaffected.

Actual behavior

Measured against 07f5893 by driving the editor mount used by the plugin tests, copying a whole paragraph:

Document text	Saved	Copied
# not a heading	\# not a heading	# not a heading
*not emphasis* tail	\*not emphasis\* tail	*not emphasis* tail
[test link](./test.html) tail	\[test link]\(./test.html) tail	[test link](./test.html) tail

A selection that is not text-only escapes correctly. The same paragraph copied together with a second paragraph, or with an emphasized span inside it, yields \# not a heading.

Pasting back into Leafdown keeps the text literal, because the HTML flavor wins there. The defect shows wherever the plain-text flavor is the one that survives.

Related context

• Related issues or pull requests: Typed link source survives the save only when it is not followed by a space #234 and fix: keep typed link source literal when a space follows it #242 fixed this class of defect on the save path; Commit typed link source to a link when the caret leaves it #239 reaches the same mismatch from inside the editor
• Documentation, decisions, or prior investigation: measured with the editor mount used by the plugin tests

Done when

• Copying a text-only selection carries the escapes the save path writes.
• Regression coverage asserts the clipboard Markdown for an escaped heading marker, emphasis marker, and inline link, alongside a selection that is not text-only.

Notes, logs, screenshots

Diagnosis

Milkdown's clipboard plugin installs clipboardTextSerializer, which returns slice.content.textBetween(...) when the slice is pure text and only otherwise builds a document and runs serializer(doc) (@milkdown/plugin-clipboard 7.21.3, lib/index.js). #242's escaping handler is installed on the serializer, so it never runs for a text-only slice. That is why adding a second paragraph or a formatted span to the selection restores the escapes.

Out of scope

• The HTML flavor, which carries the same characters and is parsed as text by the in-app ingress.
• Whether an in-app copy should preserve literalness beyond what Markdown can express, which belongs to the escape-representation spike.

[Azganoth]

Re-measured against f409cdbf, after #252 landed. The defect is unchanged — the copied column is still the document text with no escaping — but one row of the table above is now stale, and the fix is smaller than it was when this was written.

Document text	Saved	Copied
# not a heading	\# not a heading	# not a heading
*not emphasis* tail	\*not emphasis\* tail	*not emphasis* tail
[test link](./test.html) tail	\[test link](./test.html) tail	[test link](./test.html) tail

The third row's Saved value was \[test link]\(./test.html) tail. #252 removed the second escape, because a \[ already stops the link and the \( changed nothing.

The two escapes that do hold are unchanged, which is the useful part: this issue's Expected behavior asks the clipboard to carry the escapes the save path writes, and the save path now writes only the ones the text needs. Routing the text-only flavor through serializeMarkdownText gets the precise set rather than the blanket one, so the expected behavior and the acceptance criteria still read correctly as written — there is just less to reproduce.