fix(composer): preserve caret when inserting mentions mid-message

[tellaho]

Category: fix
User Impact: Users can insert mentions earlier in a draft and continue typing without the caret corrupting the rest of the message.

Problem: Caret correction ran after every document change, so typing a mention before existing text repeatedly advanced across the mention separator and interleaved spaces into the draft. Solution: Limit correction to the autocomplete settlement it was designed for, with transaction-level and browser-level regression coverage for known and unregistered mentions.

File changes

desktop/src/features/messages/lib/mentionHighlightExtension.ts
Restricts trailing-space caret advancement to an armed autocomplete settlement instead of every document change.

desktop/src/features/messages/lib/mentionHighlightExtension.test.mjs
Exercises the real ProseMirror plugin state and verifies mid-draft mention typing, unknown tokens, end-of-message typing, and completed-mention separators.

desktop/tests/e2e/mentions.spec.ts
Reproduces the reported composer workflow in Chromium and covers the same corruption path for an unregistered @token.

Reproduction steps

1. Open a channel and enter hello world in the composer.
2. Move the caret between hello and world.
3. Type @bo, select bob from autocomplete, and continue typing abc.
4. Confirm the composer reads hello @bob abc world with the caret after abc.
5. Repeat with an unregistered token such as @zzq and confirm the existing text remains intact.

Before / After

Before	After
Typing after a mid-draft mention walks the caret through the existing message.	Continued typing stays after the inserted mention.

[wesbillman]

Carl, an automated reviewer, commenting via Wes’s GitHub account.

No blocking findings. I reviewed exact head cbbe2b9dd43329cc203800a539538081d60601c0 against base 0e48ff26915aa32d5f05208847b9aba75f4f19cd.

The change removes the invalid “any document change” caret advance while preserving the editor-local autocomplete settlement path and its cancellation boundaries. Coverage exercises the pure predicate, the real ProseMirror plugin transaction flow, and Chromium behavior for both selected and unregistered mid-draft mentions.

Focused validation at the exact head passed: all 41 mention-highlight unit tests, both new Playwright regressions, and Biome on the three changed files. Desktop Core, smoke E2E, integration E2E, macOS build, and DCO CI checks are green.

[jedwards27]

:bot: Jude’s code review agent

Approved at exact head cbbe2b9dd43329cc203800a539538081d60601c0 against base 0e48ff26915aa32d5f05208847b9aba75f4f19cd.

No material findings. Independent systems/integration and product/UI/adversarial review lanes agree that the narrow predicate change in desktop/src/features/messages/lib/mentionHighlightExtension.ts preserves autocomplete settlement while preventing ordinary mid-draft document changes from advancing the caret. Deliberate ArrowLeft/click cancellation remains intact, surrounding text is preserved, and the change introduces no relay, schema, identity, persistence, dependency, Tauri, release, or accessibility-contract scope.

Exact-head validation:

• pnpm --dir desktop test: 5,354 passed, 0 failed.
• pnpm --dir desktop typecheck: passed.
• Biome on all three changed files: passed.
• Isolated Chromium E2E: both new mid-draft mention regressions passed.
• Causal mutation restoring unconditional document-change advancement made both regressions fail; restoring the exact head returned them to 2/2 passing.
• git diff --check: passed; working tree remained clean.
• Applicable exact-head CI is green, including Desktop Core, four smoke shards, two integration shards, macOS build, aggregate Desktop, and DCO.

Residual risk: no native Tauri/WKWebView journey was run because shared-machine GUI launch/focus was not authorized. Platform-specific DOM-selection behavior therefore remains unproven natively, though the production change is narrow and the causal Chromium regressions substantially constrain the risk.