[kevin9327]

Description

In CommonMark an ATX heading can end with a closing sequence of # characters. That sequence is not part of the heading text (example 71):

## Hello world ##

renders as <h2>Hello world</h2>. $convertFromMarkdownString keeps the #s as text instead. So in the editor the heading reads "Hello world ##", with the closing #s shown as part of the title.

On import, HEADING.replace now strips a trailing closing sequence from the heading's text: one or more #, optionally followed by spaces or tabs. As the spec requires, the sequence must be preceded by a space or tab, so # Hello C# keeps its #. The typing shortcut (isImport === false) is unchanged.

Test plan

New cases in LexicalMarkdown.test.ts:

• ## Hello world ## imports as an h2 reading Hello world.
• # Hello world ##### (longer sequence plus trailing spaces) imports as an h1 reading Hello world.
• # Hello C# still imports as Hello C# and round-trips.

Before

× can import "## Hello world ##"
× can import "# Hello world #####   "
Tests  2 failed | 812 passed (814)

After

pnpm exec vitest --project unit --no-watch packages/lexical-markdown

Tests  814 passed (814)

Prettier, ESLint and tsc --noEmit are clean.

AI-assisted: I directed the investigation and reviewed and verified the change myself.

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Project	Deployment	Actions	Updated
lexical	Ready	Preview	Oct 29, 2026 9:34pm UTC
lexical-playground	Ready	Preview	Oct 29, 2026 9:34pm UTC

[pkg-pr-new]

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commit: 048d6f9