Fix reading position and composer caret on channel return

[wesbillman]

Opened by Brain on Wes's behalf; implementation and commit by Carl.

Summary

• Retire a conversation's old send/reveal instruction on departure so it cannot override the saved reading position on return. Preserve fresh-send and New-message → existing-DM handoff.
• Focus the composer before selecting the draft end, then retain the successful focus handoff across development StrictMode replay. Explicit dialog focus and deliberate selections on updates still win.
• Production diff: two files, +12/−1. No cache, virtualizer, editor-adapter, timer, or persistence changes.

Draft gate

An existing WebKit no-send cold-reload check failed on this tree: tests/browser/scroll.spec.mjs:155, same message ID but 650px Y displacement (contract: <4px). It passed in the one unchanged-production baseline comparison, so this is not established as unrelated or pre-existing. No retry-to-green, relaxed assertion, or out-of-scope reflow repair. Resolve this evidence gap before ready/merge; CI and required human/code-owner approval also remain gates.

Review and validation

Checked base 14a2e7ed585b130315629ded39d1b83b05eb2a53; head ddd51669d11b65412a9025f75a60cb69e68da9b7.

• Brain independently reviewed the production/test diff against ownership, reading-position and focus contracts; no source-level blocker found. This does not waive the failing browser contract above.
• Wes confirmed correct cursor behavior in the trial worktree. Browser checks use ephemeral identities and modeled relay I/O, not live sends or packaged-native acceptance.
• Mandatory pre-commit passed. Mandatory pre-push at the committed head passed TypeScript, 405 tests / 29 files, design types and guards. Managed hooks preserved; no bypass.
• Complete composer/scroll browser files: 11/12 passed across Chromium/WebKit; failure above. The unanchored selection also ran 14 image-scroll cases, all passing.
• Complete strengthened new-message file: 4/4 passed, including a genuinely scrolled existing DM → New message → confirmed-send reveal.
• Both new regressions fail before / pass after in both engines: successful send → read older messages → leave/return preserves message/Y; returning after a successful two-agent send appends immediate keyboard input after retained mentions and preserves exact signed recipient tags.
• Behavior checks ran before commit on the reviewed diff; all seven committed file hashes match that snapshot (new-message ran before formatting-only changes). No postcommit browser rerun is claimed. Broad validation is left to existing CI.

Coverage and reproduction

Two browser cases added, none removed. Existing DM case strengthened; component coverage adds StrictMode end selection and deliberate-selection preservation, and enables StrictMode in the explicit focus-handoff guard. Browser-only justification: native DOM selection/keyboard insertion and real wheel/Virtua geometry cannot be proved by jsdom. Existing developmentReact fixture option is reused, with one opt-in agent-peer roster/profile fixture input.

# Exact affected browser files, both configured engines, without measurement dependencies:
bin/pnpm exec playwright test --config tests/browser/playwright.config.mjs \
  '(^|/)(composer-focus|scroll|new-message)\.spec\.mjs$' --no-deps --workers=1

Local macOS arm64, Chromium 153.0.8010.12 / WebKit 26.6; new scroll cases 4.8s/4.9s, caret cases 2.0s/2.4s. No CI performance claim.

Originating conversation: buzz://message?channel=2174385b-ca3b-4b31-8ad6-846f10fae18f&id=81c4995799ba210c745a999187c547e4fc87a1cbe544b10ef0f6df5206f5de03

[kalvinnchau]

🤖 No new actionable defects found, but I am not recommending approval yet. The reported WebKit cold-reload result displaced the saved reading anchor by 650px against the <4px contract (tests/browser/scroll.spec.mjs:155). A subsequent exact-head Chromium/WebKit run passed 16/16 tests, but does not explain that failure. Please isolate the cause or explicitly accept the residual risk before merging; a causal production regression has not been established.