Kyle Carberry 1a87e74574 fix(site): prevent race conditions when switching chats on agents page (#22404)
## Problem

When switching between chats on the agents page, stream parts could be
lost or applied to the wrong chat due to several race conditions in
`ChatContext.ts`:

1. **`startTransition` deferred parts escape cleanup** —
`startTransition(() => store.applyMessageParts(parts))` defers the state
update. If a chat switch happens between `flushMessageParts` being
called and the transition executing, old-chat parts could apply after
`resetTransientState()` has already cleared stream state for the new
chat.

2. **`message` event has no `chat_id` filter** — Unlike `message_part`,
`queue_update`, and `status` events, the `message` event handler did not
check `streamEvent.chat_id`. While the server-scoped WebSocket makes
this safe in practice, it's an inconsistency in defensive programming.

3. **Brief stale message window on switch** — Between `chatID` changing
and `replaceMessages()` firing (after the query resolves), the store
held old-chat messages while the new WebSocket was already connected.

## Changes

### `ChatContext.ts`
- Added `activeChatIDRef` to track the currently active chat ID
- Guard `startTransition` callback: check `activeChatIDRef` before
applying message parts, discarding them if the chat has switched
- Added `chat_id` filter to `message` event handler, matching the
pattern used by all other event types
- Added `store.replaceMessages([])` to the chatID-change effect so
messages are cleared immediately on switch

### `ChatContext.test.tsx`
Four new tests covering the chat-switch lifecycle:
- WebSocket closure and state reset when chatID changes
- `message` event filtering by `chat_id`
- `startTransition` deferred parts discarded after switch
- Messages cleared immediately before new query resolves

All 13 tests pass (8 existing + 4 new + 1 existing).
2026-02-27 15:33:16 -05:00
2022-04-04 11:55:06 -05:00

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