Summary

On the desktop app (reproduced on macOS), sending a message while the agent is busy queues a follow-up — but then the Stop button never actually interrupts the running turn. Clicking the arrow/Stop button after queuing keeps the agent running on the queued prompt instead of halting.

Root cause

Two cooperating bits of logic in apps/desktop/src/app/chat/composer/:

1. submitDraft Stop branch. When busy with an empty composer (the state right after a follow-up has been queued and the composer cleared), the primary button shows the Stop square. But clicking it ran interruptAndSendNextQueued() — cancel the turn and immediately re-send the head of the queue. So Stop never stopped; it just swapped which prompt the agent was running.

2. Auto-drain on settle. The busy: true → false effect unconditionally re-sent the next queued prompt. Even a pure onCancel() would flip busy false and the queue would auto-fire. The user could never halt while anything was queued.

Net effect: the Stop affordance lied — it rendered as a Stop square but behaved as "interrupt, then keep going."

Fix

• Stop button (busy + empty composer) is now a pure interrupt via onCancel(). It no longer hijacks the queue.
• An explicit interrupt latches userInterruptedRef so the busy → false auto-drain skips exactly one drain. Queued turns are preserved; the user resumes them deliberately — Cmd/Ctrl+K, Enter on an empty composer, or the per-row "send now" arrow — matching the documented affordances (Esc = cancel run, Cmd+K = send next queued).
• The settle decision is extracted into a pure shouldAutoDrainOnSettle() helper in composer-queue.ts so the invariant is explicit and testable.
• Removed the now-unused interruptAndSendNextQueued.

Enter while busy and the Stop button now behave identically (both interrupt), which is the intended "cancel run" semantics.

Test plan

New unit tests in composer-queue.test.ts cover shouldAutoDrainOnSettle:

• drains the next queued prompt when a turn completes naturally
• does not drain when the user explicitly interrupted (Stop) — regression guard
• does not drain on empty queue
• ignores steady busy / busy-entry / steady-idle (only a true→false transition triggers)

✓ src/store/composer-queue.test.ts (12 tests)
  Test Files  1 passed (1)
       Tests  12 passed (12)

Manual repro before/after on the desktop composer:

1. Send a message → agent busy.
2. Type a second message, click the button → it queues (Layers icon), composer clears.
3. Click the Stop square.
   • Before: agent keeps running on the queued prompt.
   • After: turn halts; the queued message stays in the queue for the user to send when ready.