Thanks for splitting this out of #1579 — clean focused diff, mergeable against main, and the core mechanism is right.

One blocker before I can merge, though. The flag gets stuck true on the most common path, not just an edge case.

Sequence that breaks (any session switch, not just abort-then-switch):

1. Previous turn finishes normally → runTurn's finally runs → tab.aborter = null, tab.switching = false (already).
2. User clicks a different session → session_load handler runs:

   tab.switching = true;
   abortTurn(tab);  // tab.aborter?.abort() is a no-op — aborter is null
   ...

3. tab.switching is now true. No runTurn is in flight, so no finally fires to reset it.
4. User types the first message in the new session → new runTurn runs → finally:

   if (!tab.switching) { /* … */ }   // false → all wrap-up events skipped
   tab.switching = false;

5. Every terminal event for that first new-session turn is suppressed: $turn_complete, emitSessions, emitBalance, $plan_cleared, the QQ channel response.

The user-visible symptom is worse than #1217 was: the UI sits on "thinking" forever because $turn_complete never arrives, balance/session list don't refresh, and the QQ integration drops the first reply. This isn't an edge case — it's the standard "switch session, then send a message" flow.

Minimal fix

Only set the flag when there's actually a live turn to abort. Two-line change at both call sites:

// session_load and new_chat handlers
if (tab.aborter) tab.switching = true;
abortTurn(tab);

That keeps the rest of the PR identical. If you'd rather avoid the shared mutable flag entirely, the cleaner alternative is to snapshot the session in runTurn and compare in finally:

async function runTurn(tab: Tab, text: string, fromQQ = false): Promise<void> {
  if (!tab.runtime) return;
  const startSession = tab.currentSession;
  ...
  finally {
    tab.aborter = null;
    if (tab.currentSession === startSession) {
      // emit $turn_complete / emitSessions / emitBalance / $plan_cleared / QQ
    }
  }
}

Then drop the tab.switching field and the two tab.switching = true lines entirely. No shared state, no reset, no stuck-flag class of bug. Whichever you prefer.

Push a fixup and I'll merge.

Applied your suggested fix:

\\ s
// Only set switching flag when there's a live turn to abort
if (tab.aborter) tab.switching = true;
abortTurn(tab);
\\

This prevents the flag from getting stuck when no turn is in flight (the common session-switch path). Verified locally: lint ✅, typecheck ✅.

Fixup looks right — gated on tab.aborter so the flag can't stick true on the common path. Merging.