The serializer escapes tildes that cannot form a strikethrough where they sit

[Azganoth]

Summary

Saving escapes every ~ in text on the possibility that it could be a strikethrough marker rather than on whether a marker can form where it sits. #252 made that check precise for *, _, [, and (; tildes reach the same seam and were left outside its scope. This is the largest escape class left in the corpus.

Steps to reproduce

1. Open a document containing ~opening-only single-tilde strikethrough.
2. Save it.
3. Read the file.

Expected behavior

~opening-only single-tilde strikethrough. One tilde run with no counterpart cannot form a strikethrough, so no escape is needed.

Text that would parse differently without an escape keeps it. Both directions matter, as in #252: a run that can open beside a run that can close must stay escaped.

Actual behavior

Measured against 9626471d by driving the editor mount used by the plugin tests. Every row below is stable across a second save, so the corpus guard is green on all of them:

Document text	Saved
~opening-only single-tilde strikethrough	\~opening-only single-tilde strikethrough
closing-only double-tilde strikethrough~~	closing-only double-tilde strikethrough\~\~
~ opening space~ and ~closing space ~ remain literal.	\~ opening space\~ and \~closing space \~ remain literal.
Inline ~~~three tildes do not strike~~~ remains literal.	Inline \~\~\~three tildes do not strike\~\~\~ remains literal.
Empty candidates: ~~ and ~~~~ remain literal inline text.	Empty candidates: \~\~ and \~\~\~\~ remain literal inline text.
This ~~does not cross / a paragraph boundary~~. in two paragraphs	both runs escaped

corpus/gfm/strikethrough.md carries 54 backslashes on save and 16 of its lines differ from source by escaping alone. It is the only scoped corpus file whose escaping #252 left untouched.

Related context

• Related issues or pull requests: Preserve the authored form of a Markdown file on save #251 is the parent; The serializer escapes characters that cannot be syntax where they sit #252 owns the same mechanism for the core attention and bracket characters and deliberately excluded ~; Determine whether the Markdown corpus can drive automated round-trip tests #135 measured the identity gap; test: assert corpus round-trip convergence for supported syntax #249 owns the corpus guard this tightens.
• Documentation, decisions, or prior investigation: the Outcome sections of Determine whether the Markdown corpus can drive automated round-trip tests #135 and How a Markdown escape should be represented in the editor #245.

Done when

• The cases above save without an escape they do not need.
• A tilde run that can reach a counterpart keeps its escape, including \~not single-tilde strikethrough~ and \~\~not double-tilde strikethrough~~.
• A tilde run that could open a fenced code block at a line start keeps its escape.
• Regression coverage asserts both directions, alongside the Escape precision cases The serializer escapes characters that cannot be syntax where they sit #252 added.
• corpus/gfm/strikethrough.md round-trips byte-identically, or the reason it cannot is recorded.

Notes, logs, screenshots

Diagnosis

mdast-util-gfm-strikethrough contributes {character: '~', inConstruct: 'phrasing', notInConstruct: [...]} to state.unsafe, so the escape is emitted through the same state.safe call that serializeMarkdownText in src/features/editor/utils/markdownText.ts already post-processes. That function's attention pass reads ATTENTION_CHARACTERS, which holds * and _ only.

Implementation direction

GFM strikethrough uses the same left- and right-flanking rules as *, so canOpen is left-flanking and canClose is right-flanking, and the existing run analysis carries over unchanged. Three differences to account for:

• Only a run of one or two tildes can form a strikethrough, so a run of three or more never needs the escape at all. The corpus exercises this directly.
• Strikethrough does not cross a paragraph boundary. The existing sibling scan already respects that, because it never leaves the phrasing parent.
• The core unsafe list also carries {atBreak: true, character: '~'} for a tilde code fence, so the opensBlockConstruct guard must cover ~ rather than being skipped for it.

Out of scope

• The autolink-literal escapes \@, \., and \:, which belong to A bare URL is escaped on save with an escape that does not hold #241.
• ~strike~ rewritten to ~~strike~~ on save. That is a marker rewrite rather than an escape, and Preserve the authored form of a Markdown file on save #251's Scope already lists it.