feat(web): add surround selection in composer

[mbuvarp]

What

Add surround-selection support in the composer so that when text is selected and the user types a supported surround symbol, the selection is wrapped instead of replaced.

This also includes dead-key backtick support, so layouts that produce ` through composition can still surround the selected text.

Currently applies to these symbols: (, [, {, ', ", `, <, «, *, _

Why

I understand this is a feature and not a bug fix, and might land outside of what you are likely to accept re: the contribution guidelines. But it's something I always miss in editors when they don't have it, since I habitually select text and press parenthesis, quote or whatever.

If this is something that should be opt-in behind a setting, I can add that as well.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included a video for animation/interaction changes

Video

t3code-surround-selection.mov

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Note

Medium Risk
Intercepts keydown/beforeinput/composition events in the Lexical composer to rewrite prompt text and selection, which can regress typing/IME behavior around mentions and inline tokens despite added tests.

Overview
Composer typing enhancement: when text is selected in the chat composer and the user types a supported opening symbol (e.g. (, ", `, *), the selection is wrapped with the matching close symbol and the inner selection is preserved for repeated wrapping.

Adds a new ComposerSurroundSelectionPlugin that hooks keydown/beforeinput/input/compositionend, skips wrapping when the selection is collapsed, touches inline tokens, or crosses mention-boundary whitespace, and merges dead-key backtick composition into a single undo step.

Extends mention parsing utilities with selectionTouchesMentionBoundary and fixes splitPromptIntoComposerSegments to preserve consecutive terminal-context placeholders; adds browser/integration tests covering wrapping, backward selections, dead-key composition, and mention edge cases.

^{Reviewed by Cursor Bugbot for commit 8daa878. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Add surround selection to the composer for wrapping selected text with paired symbols

• Adds ComposerSurroundSelectionPlugin in ComposerPromptEditor.tsx that intercepts beforeinput/keydown events: when a recognized open symbol (e.g. (, [, ', *, «) is typed with a non-collapsed selection, the plugin wraps the selected text with the corresponding pair and keeps the inner text selected for successive wrapping.
• Selections that touch mention boundary whitespace or inline tokens fall back to normal replacement behavior.
• Adds selectionTouchesMentionBoundary in composer-editor-mentions.ts to detect when a selection includes mention-adjacent whitespace.
• Dead-key backtick composition is detected and transformed into a backtick surround in a single undo step via HISTORY_MERGE_TAG.
• Fixes splitPromptIntoComposerSegments to preserve consecutive terminal-context placeholders as distinct segments.

^{Macroscope summarized 8daa878.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces a new user-facing feature: surround selection behavior in the composer that wraps selected text with matching bracket/quote pairs. The change adds ~260 lines of new plugin code with complex event handling (keyboard, input, composition events) and edge case logic for mentions. New features introducing new UI behavior warrant human review regardless of scope.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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