Detect account switcher race / session flip mid-compose and abort send

[Trace824]

Summary

During compose, SideNav / account session can flip (user or another client). Sending without detecting the flip risks posting as the newly active account. Contended shared Chrome sessions amplify this.

Repro

• Context: Compose open; second actor (or UI) switches account via SideNav.
• Steps:
  1. Start compose as account A.
  2. Trigger account switch to B mid-compose (SideNav).
  3. Click post/reply.
• Observed: post goes out as B; SideNav may have shown A earlier in the flow. TAB_NOT_OWNED can also appear when another agent owns the tab (extension/utils/agent-tab-resolver.js).

Code pointers (verified against /workspace/FSB)

• extension/site-guides/social/twitter.js — account switcher + tweet button selectors.
• extension/utils/agent-tab-resolver.js — TAB_NOT_OWNED.
• extension/content/actions.js — click handlers for send; likely area to snapshot identity at compose-open and re-check at send.
• mcp/README.md — multi-agent ownership (TAB_NOT_OWNED).

Proposed fix

Snapshot account identity when compose is focused; on tweet-button click, re-read identity and abort if changed. Optionally combine with tab-ownership errors so soft contention fails closed.

Acceptance / regression

• Mid-compose switch → send aborted with clear error.
• No twin or wrong-account publish in the race window.

Owner

extension

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Priority: P1 · Owner: extension