The serializer escapes a hyphen run that cannot open a block where it sits

[Azganoth]

Summary

Saving escapes a -- at the start of a line, where no block a hyphen can open will form. Two hyphens are too few for a thematic break, a bullet marker is one hyphen followed by whitespace, and a setext underline needs a paragraph line directly above it inside the same block. The line is literal text as written and as escaped, so the file gains a backslash the author did not write and the document is unchanged.

corpus/commonmark/blocks.md carries the line under the heading Too few or unsupported markers remain text, which is the property being measured. The ++ + two lines below it already stays bare.

Steps to reproduce

1. Open a document whose own line holds --.
2. Save it without editing.
3. Read the file.

Expected behavior

--, unchanged.

Actual behavior

Measured against 8bc36754 by driving the editor mount used by the plugin tests, on corpus/commonmark/blocks.md line 20.

Document	Saved
--	\--
text / blank / --	-- still escaped
> --	> \--

The neighbouring cases are all left alone, which is what makes the escape inconsistent rather than a house style:

Document	Saved
-	unchanged
---	unchanged, a thematic break
+, ++ +, *	unchanged
- x	unchanged, a bullet list
a -- b	unchanged, not at a line start
- --	unchanged, inside a list item

The last row is the sharpest: the same two hyphens are escaped at the start of a top-level line and inside a blockquote, but not inside a list item.

Related context

• Related issues or pull requests: Preserve the authored form of a Markdown file on save #251 is the parent. The serializer escapes markers where the construct they open cannot form #301 is the closest precedent, relaxing block markers escaped where the block cannot start; The serializer escapes a marker run too short to open a thematic break #373 is the sibling class for an asterisk or underscore run, which reaches a different check.
• Documentation, decisions, or prior investigation: docs/decisions.md under Preserve the form a file was written in; corpus/commonmark/blocks.md under Too few or unsupported markers remain text.

Done when

• A hyphen run at a line start that can open no block is written without an escape, at top level and inside a blockquote.
• A hyphen that can open one keeps what it needs: --- still writes a thematic break, - x still writes a bullet item, and a setext underline under a paragraph is still written so it reloads as a heading.
• Regression coverage asserts the saved bytes and the document the saved file reopens as, since closure already holds and cannot catch this.

Notes, logs, screenshots

Diagnosis

No relaxation pass reaches a hyphen. relaxBlockMarkerEscapes branches only on #, the ordered-list markers, and |, and ATTENTION_CHARACTERS is *_~, so a hyphen never reaches opensBlockConstruct either. The escape state.safe emits is never revisited.

That check is the model for what a hyphen branch needs to answer, and it is more than the thematic break: a hyphen also opens a bullet item and, unlike an asterisk or an underscore, a setext underline. The setext case is the one a fix must not relax away, since dropping the escape there would change what the file reloads as rather than only how it is spelled.

Out of scope

• The ** on the line above in the same file, which is The serializer escapes a marker run too short to open a thematic break #373.
• The backtick escapes under Block structure takes precedence over inline parsing in the same file, which are an inline construct rather than a block marker.