Summary

This fixes the remaining composer @-completion bug described in pingdotgg/t3code#291.

When @ completion is started inside existing whitespace, especially in prompts that already contain mention pills, the composer could mix collapsed mention-aware offsets with expanded text offsets. That caused three visible failures:

• the completion menu could dismiss while typing even though the query was still valid
• selecting a completion could leave an extra trailing space
• the caret could land farther right than expected after insertion

Repro

1. In the composer, insert a file mention so there is already at least one mention pill in the prompt, for example @AGENTS.md.
2. Continue typing normal text and leave two spaces somewhere later in the prompt.
3. Move the cursor into that double-space gap so there is one space before the cursor and one space at the cursor position.
4. Type @.
5. Type one or two more characters so the menu filters to matching results.
6. Select a completion.

Root Cause

The composer still had a few paths that were not consistent about cursor coordinate systems:

• mention-aware composer state uses collapsed offsets, where a rendered mention pill counts as length 1
• Lexical selection state and raw prompt text use expanded offsets, where @path/to/file counts by its full text length

That mismatch showed up in trigger detection, cursor restoration, prompt replacement, and a few state resets. In the whitespace-started case, it made the active trigger drift out of sync with what the user was actually editing.

Fix

• add explicit collapsed-cursor helpers in composer-logic.ts, including clampCollapsedComposerCursor and collapseExpandedComposerCursor
• keep ComposerPromptEditor snapshots in both coordinate systems so callers can use the right cursor form for the right job
• normalize ChatView state updates, trigger detection, and cursor restoration around collapsed cursor state plus expanded trigger positions
• when inserting replacements that already end with a space, consume an existing space at the cursor instead of leaving a doubled trailing space
• keep focus and caret placement aligned after insertions, retries, pending-question handoff, and other composer state resets

Result

After this change:

• the completion menu stays open while the typed query still matches results
• selecting a completion replaces the query cleanly
• the inserted mention ends with exactly one trailing space
• the caret lands immediately after that trailing space

Testing

• bun lint
• bun typecheck
• added focused unit coverage for collapsed/expanded cursor conversion and mention-aware clamping in apps/web/src/composer-logic.test.ts