fix: composer @file autocomplete cursor and spacing bugs

[chuks-qua]

Closes #291
Closes #922

What Changed

Three fixes for the composer's @file autocomplete.

Mid-text @ trigger detection:
ComposerPromptEditor now computes an expanded cursor from the Lexical node tree and passes it through onChange and readSnapshot. ChatView uses this expanded cursor in detectComposerTrigger instead of the regex-based expandCollapsedComposerCursor.

Double space after completion:
When the replacement (@AGENTS.md ) already ends with a space and a space exists at the cursor, the insertion now consumes the existing space instead of leaving two.

Caret lands at end of line after completion:
applyPromptReplacement returned an expanded cursor from replaceTextRange but stored it as a collapsed cursor. All setComposerCursor call sites now use the correct coordinate system.

Code changes:

• Add getExpandedAbsoluteOffsetForPoint and $readExpandedSelectionOffsetFromEditorState to ComposerPromptEditor.tsx
• Pass expandedCursor through the onChange callback and readSnapshot interface
• Add collapseExpandedComposerCursor and export clampCollapsedComposerCursor in composer-logic.ts
• Add extendReplacementRangeForTrailingSpace to consume trailing spaces during completion
• Fix all setComposerCursor call sites to store collapsed offsets
• Add unit tests for collapsed/expanded cursor conversion and trailing space consumption

Why

expandCollapsedComposerCursor uses MENTION_TOKEN_REGEX to locate mentions in raw text. When a user types @ before an existing word (e.g., cursor between inspect and in this sentence), the regex matches @in as a mention and shifts the cursor past it. detectComposerTrigger then receives the wrong offset and returns null, so the autocomplete popup never appears.

The Lexical editor already tracks which nodes are ComposerMentionNode instances. Reading the expanded cursor directly from the node tree eliminates the regex false-match problem.

The other two bugs come from mixing collapsed and expanded cursor offsets in ChatView and ComposerPromptEditor. Collapsed offsets count a mention pill as 1 character; expanded offsets count by full text length (@AGENTS.md = 11). Passing an expanded offset where collapsed is expected inflates the cursor position, and failing to consume an existing trailing space doubles the gap.

UI Changes

Before

composer-before-fix.mp4

After

compose-after-fix.mp4

In the After video, also notice: after selecting a completion mid-text, the cursor lands immediately after the inserted mention (not at the end of the line), and there is exactly one space between the mention and the next word.

Checklist

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

Note

Fix @file autocomplete trigger detection when cursor is mid-prompt with trailing text

• The composer now tracks both a collapsed cursor (where mentions count as 1 character) and an expanded cursor (full mention text length), using the expanded cursor for trigger detection and the collapsed cursor for focus/selection operations.
• Adds collapseExpandedComposerCursor and clampCollapsedComposerCursor exports to composer-logic.ts to convert between cursor representations.
• ComposerPromptEditor.tsx now computes and reports both cursor forms to callers via an extended onChange signature and readSnapshot return value.
• Adds extendReplacementRangeForTrailingSpace in ChatView.tsx so inserting a path or /model token that ends with a space consumes any existing trailing space at the insertion point, preventing double spaces.
• Behavioral Change: trigger detection now uses the expanded cursor offset, which fixes false-positive matches around mentions but changes how cursor position is computed during autocomplete.

^{Macroscope summarized 28a40d7.}

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[chuks-qua]

authorization is needed for redeployment

[mwolson]

Hi, thanks for looking into the issue I filed for #291 . I took a brief look as of 8f3b32a the likely remaining items I have are:

• applyPromptReplacement() still doesn’t consume an existing trailing space under the cursor, and it still feeds the replacement cursor back through the composer as if expanded/raw-text offsets were mention-aware. That seems likely to leave the “double space after insertion” / “caret lands too far right” part unresolved.
• A focused regression test for the exact whitespace-started selection flow from Composer @ completion started in whitespace can dismiss early and insert with wrong spacing #291 might be nice to have

[chuks-qua]

Hi, thanks for looking into the issue I filed for #291 . I took a brief look as of 8f3b32a the likely remaining items I have are:

• applyPromptReplacement() still doesn’t consume an existing trailing space under the cursor, and it still feeds the replacement cursor back through the composer as if expanded/raw-text offsets were mention-aware. That seems likely to leave the “double space after insertion” / “caret lands too far right” part unresolved.
• A focused regression test for the exact whitespace-started selection flow from Composer @ completion started in whitespace can dismiss early and insert with wrong spacing #291 might be nice to have

Hey, thanks for pointing this out, i am looking into it now.

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Edit

Looked into it and now i follow what you are saying.
Trailing Space
When you select a completion in a double-space gap:
and_then_|@AG|__summarize
The replacement @AGENTS.md already has a trailing space, but the existing space after the cursor stays too:
and_then_@AGENTS.md__surmmarize

Note

used _ for spaces and | to group the suggestion trigger

Cursor after insertion
When a mention pill like @AGENTS.md already exist earlier in the prompt, the code stores the wrong cursor position after inerting a new completion. the pill counts as 1 char internally but 11 chars in the raw text. applyPromptReplacement stores the raw text offset without converting it, so the caret jumps to the end of the line instead of landing right after the insertion.

I am implementing the fixes.