[Bug]: Rich text composer consumes typed underscores in snake_case text, including inside inline code #19169

Area

apps/web (message composer, rich text mode)

Summary

In the rich text composer, typing a second _ on a line consumes both underscores and italicizes everything between them. It happens mid-word in snake_case text such as node_modules, and it reaches back into an already completed inline code span. The underscores do not come back.

Steps to reproduce

Rich text composer enabled. Type every character; do not paste.

Case 1: plain text

1. Start with an empty composer.
2. Type:

   clean _build and node
   

3. Type _
4. Press Right, then type modules

Case 2: inline code

1. Start with an empty composer.
2. Type the line below. The first backtick pair becomes a code span showing _build; the last backtick stays open.

   clean `_build` and `node
   

3. Type _
4. Type, with no leading space:

   modules`
   

Expected behavior

The composer keeps exactly what was typed, with no italics.

Case 1:

clean _build and node_modules

Case 2, shown as two inline code spans:

clean `_build` and `node_modules`

An intraword _ does not close emphasis in CommonMark, and inline code content is literal.

Actual behavior

Case 1

• After step 3, both underscores are consumed and replaced with italic formatting on build and node.
• After step 4, the composer shows clean, italic build and node, then plain modules. The underscores are still gone.

Case 2

• After step 3, the underscore inside the completed code span is consumed along with the one just typed. The span shows build, and italics run from inside it through node.
• After step 4, the composer shows an italic code span build, italic and, a code span holding italic node plus a literal backtick, and a separate non-italic code span holding modules plus a literal backtick. No underscore is left.

Impact

Prompts for coding agents are full of leading-underscore names, snake_case identifiers, and paths. The composer changes them during typing, and the change is easy to miss. Wrapping the name in inline code, the usual way to protect a literal, does not prevent it.

Version or commit

0.0.46-nightly.20261013.3066 (tag v0.0.46-nightly.20261013.3066, commit 98a6246d07624432a6e6d0ab4fb87bede90a49a6)

Environment

macOS desktop app. No provider or model involved; nothing needs to be sent.

Source notes

Unconfirmed reading of the source at that commit:

• Italic rule: rich mode leaves StarterKit's stock italic enabled (ComposerPromptEditorTiptap.tsx#L1063). Its Tiptap 3.31.3 underscore input rule fires on the closing _ and deletes both delimiters.
• Code guard: Tiptap's markInputRule skips matches overlapping a mark that excludes italic, and excludes: "code" in ComposerCodeExtension removes that guard (composer-rich-text-doc.ts#L62-L67). This is why the rule reaches into inline code.
• Serializer: RICH_TEXT_DELIMITERS.italic is * (composer-rich-text.ts#L20-L25), so the italic run is written back with asterisks.