[nakultt]

Description

CommonMark treats everything after an opening code fence, trimmed, as the
info string, and its first word is the language. That holds whether or not
the block is closed: an unclosed fence runs to the end of the document.

Lexical read the language only when it came right after the backticks, and
kept the rest of the line out of the code only when the block was closed.
So ``` js imported with no language and js as the first line of
code, and an unclosed ```js title=x put title=x into the code.
The info string then stayed in the code on every save.

This change:

• lets CODE_START_REGEX accept whitespace between the fence and the
  language, so ``` js is a js block;
• reads the rest of an unclosed fence's opening line as metadata, the same
  way a closed fence's is read, and keeps it on export
  (```js title=x).

A closed ```js title="x" fence, ```js with nothing after it,
a bare ``` fence and the single-line ```code``` form are
unchanged. ``` code / more / ``` now becomes language
code with code more, as in CommonMark, so the tests that asserted the
old behaviour are updated.

Closes #9406

Test plan

Before

pnpm exec vitest --project unit --no-watch packages/lexical-markdown, with
the updated tests and the original MarkdownTransformers.ts:

     × treats text after the opening fence as the info string 16ms
     × keeps a blank first line of an unterminated fence with an info string 5ms
     × reads a language that follows whitespace after the fence 19ms
     × keeps the metadata of a fence with whitespace before the language 6ms
     × reads the info string of an unterminated fence 4ms
     × reads the language of an unterminated fence after whitespace 3ms
     × can import "```javascript Incomplete tag" 14ms
     × can import "```javascript Incomplete multiline\n\nTag" 4ms
 Test Files  3 failed | 5 passed (8)
      Tests  8 failed | 690 passed (698)

After

 Test Files  8 passed (8)
      Tests  698 passed (698)

pnpm exec vitest --project unit --no-watch packages/lexical-markdown packages/lexical-mdast packages/lexical-playground/__tests__/unit:

 Test Files  45 passed (45)
      Tests  1246 passed (1246)

tsc, ESLint and Prettier pass on the changed files. The Markdown shortcut
uses the same regular expression, and typing ``` plus a space
still gives the same match, but this was not tried in a browser, and the
E2E suites were not run.

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Project	Deployment	Actions	Updated
lexical	Ready	Preview	Oct 28, 2026 9:17am UTC
lexical-playground	Ready	Preview	Oct 28, 2026 9:17am UTC