fix(desktop): keep the draft space when typing right after a mention pick

[matt2e]

Problem

Desktop Smoke E2E flaked on mentions.spec.ts ("typing a mention before existing text does not interleave spaces"): picking a suggestion mid-draft and typing immediately could swallow the draft's own space, producing hello @bob abcworld instead of hello @bob a world.

With hello world drafted and @bo typed after hello, picking the suggestion leaves hello @bob world — the mention's trailing space followed by the draft's space. Chromium reconciles that whitespace run by re-emitting it as a replacement (replace(" " -> " a")), anchored at either edge of the run. The settling-time text-input redirect only recognized a caret or a rewrite anchored at the chip edge, so the other shapes fell through to ProseMirror's destructive default and ate the separator.

The race pre-dates this branch and reproduces on main.

Fix

mentionHighlightExtension.ts now redirects on the invariant instead of on a specific rewrite shape. While autocomplete is settling the user cannot have a selection (a selection cancels settlement), so a whitespace-only replacement adjacent to a mention's trailing space is always the browser normalizing whitespace — never an intentional delete. The handler keeps every space already in the document and inserts only the typed characters after the token, whichever edge the rewrite is anchored at. Non-whitespace replacements still fall through as real edits.

insertPosForMentionTextInput / mentionTextInputInsertPos become insertionForMentionTextInput / mentionTextInputInsertion, returning {insertAt, text} so the handler can rewrite the payload, not just the position.

Tests

• Helper-level cases for all four anchors (caret at chip edge, caret past the trailing space, run rewritten from the chip edge, draft space rewritten alone), plus space/NBSP variants and a non-whitespace replacement that must be left alone.
• A plugin-level test drives the real handleTextInput prop and falls back to ProseMirror's default when it declines — exactly as the browser does with the return value — so it fails against the previous production code rather than depending on winning a timing race in a headless browser.

Desktop unit suite passes on pre-push (5558/5558).

[jedwards27]

:bot: Jude’s code review agent

No unresolved author-actionable defect found at exact head 272257967f67f6cf80653fa23e35ff0e40c07acc (base db514b153a8cb17631d6e80b91c95fdc45deb147).

The change is narrowly scoped to the per-editor post-autocomplete settlement path. It preserves the existing draft separator across Chromium/ProseMirror whitespace-rewrite shapes, inserts only the stripped incoming payload, and cancels on explicit selection/navigation so main/thread editors cannot share stale settlement state. Both assigned lanes independently cleared the change.

Validation on the exact head:

• Desktop package: 5,558/5,558 passed
• Focused plugin suite: 43/43 passed; removing the new look-behind made the intended regressions fail
• Rebuilt exact mention journey: 10/10 passed in the systems lane
• Product lane: 50/50 focused E2E repetitions across routing, ArrowLeft, pointer chip-edge, mid-draft rapid typing, and Enter selection; additional spacing and punctuation probes passed 5/5 each
• Desktop check, typecheck, build, and git diff --check passed
• Required GitHub Desktop checks are green: Core, macOS build, all Smoke shards, and both Integration shards

Author action: none.

Verification owner: reviewer/tooling for optional packaged Tauri/WKWebView IME, accessibility, and real thread-panel observation. Those are confidence gaps, not evidence of a defect. CI owns continued gate health.

Residual risk is limited to native IME/AT and thread-panel observation not exercised in this review; the extension’s per-editor isolation is covered in unit tests.

[jedwards27]

:bot: Jude’s code review agent

No unresolved author-actionable defect found at exact head 272257967f67f6cf80653fa23e35ff0e40c07acc (base db514b153a8cb17631d6e80b91c95fdc45deb147).

The change is narrowly scoped to the per-editor post-autocomplete settlement path. It preserves the existing draft separator across Chromium/ProseMirror whitespace-rewrite shapes, inserts only the stripped incoming payload, and cancels on explicit selection/navigation so main/thread editors cannot share stale settlement state. Both assigned lanes independently cleared the change.

Validation on the exact head:

• Desktop package: 5,558/5,558 passed
• Focused plugin suite: 43/43 passed; removing the new look-behind made the intended regressions fail
• Rebuilt exact mention journey: 10/10 passed in the systems lane
• Product lane: 50/50 focused E2E repetitions across routing, ArrowLeft, pointer chip-edge, mid-draft rapid typing, and Enter selection; additional spacing and punctuation probes passed 5/5 each
• Desktop check, typecheck, build, and git diff --check passed
• Required GitHub Desktop checks are green: Core, macOS build, all Smoke shards, and both Integration shards

Author action: none.

Verification owner: reviewer/tooling for optional packaged Tauri/WKWebView IME, accessibility, and real thread-panel observation. Those are confidence gaps, not evidence of a defect. CI owns continued gate health.

Residual risk is limited to native IME/AT and thread-panel observation not exercised in this review; the extension’s per-editor isolation is covered in unit tests.

Duplicate submission from CLI delivery; canonical approval is review 5030502975.