stacklok / toolhive-studio / 25850508827

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feat(playground): queue messages while a response is streaming (#2246)

* refactor(playground): extract composer handle hook, tighten rewind return type

Address two nits from #2241 review:
- `onRewindAndResend`'s return type is now `Promise<void>` — no caller
  reads the underlying `sendMessage` result.
- The imperative-handle registration `useEffect` moves into a new
  `useComposerHandle` hook for readability and direct testing.

* feat(playground): queue messages while a response is streaming

When the user submits a message while the assistant is still streaming
a response, the renderer holds it as a per-thread queued message
(single slot, in-memory) and auto-fires it when the current stream
completes. A small chip above the composer surfaces the queued state
with a cancel button. The submit button morphs into a send-style icon
when the composer has text during streaming, distinguishing the queue
action from the existing stop behavior.

Mutually exclusive with the rewind-and-retry path from #2241 — when
editing the last user message during streaming, that flow still wins.
The queue is cleared on thread switch and on cancel.

* refactor(playground): compact verbose comments in queue-while-streaming code

Trim multi-paragraph JSDoc and inline explainers down to single-line
intent statements. Keep only the comments whose "why" isn't obvious
from the code itself (the rewind path bypassing the queue intercept,
the streaming→error skip on auto-flush). No behavior change.

* fix(playground): prevent queue auto-flush into wrong thread on switch

Cross-thread leak from #2246 review: when the user queued a message in
thread A and switched to thread B before A's stream finished, the
auto-flush effect could fire A's queued payload into B's `useChat`
instance. The thread-clear `useEffect` ran on the same render as the
auto-flush — both see the stale `queuedMessage`, and the prev-status
ref still held A's `'streaming'` so the streaming→ready transition
falsely ... (continued)

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