## Release v0.51.265 — Release IG (stage-r15)
Un-held: owner-aware `cancelStream()` (#3344) — author addressed the active-session SSE-settle gap.
### Fixed
| Issue/PR | Author | Fix |
|----------|--------|-----|
| #3344 | @franksong2702 | Stop/Cancel no longer leaves the UI falsely idle, drops the cancellation transcript, or leaks the old stream's tokens. `cancelStream()` is owner- + terminal-settle-aware: **active session** → leave the SSE open so the backend terminal `cancel` event clears INFLIGHT / renders "Task cancelled" / refreshes sidebar; **stale owner** (`activeStreamId!==streamId`) → tear down the SSE; local clear only on exact ownership (no null-window clobber of a turn started mid-cancel). |
### Un-hold note
Held twice earlier: (1) a null-window clobber (cleared busy on `!S.activeStreamId`, hitting a queued new turn), (2) the blocker — it called `closeLiveStream()` on the active session, killing the SSE before the terminal cancel event could settle. The author's rework fixes both (clear requires `activeStreamId===streamId`; SSE closed only for the stale-owner path). Codex confirmed the backend reliably emits the terminal cancel SSE frame the new design relies on (no busy-hang risk).
### Gate
- Full pytest suite: **7742 passed, 0 failed**
- ESLint: CLEAN · ruff: CLEAN · browser-smoke: CLEAN
- Codex (regression): **SAFE TO SHIP** — verified active cancel keeps the SSE for settle, the terminal event reliably fires, cancelled:false clears only on exact ownership, stale-owner teardown + network-error paths consistent.
Co-authored-by: franksong2702 <franksong2702@users.noreply.github.com>
