fix: deliver missed messages on SSE reconnect, preserve scroll position

[dwb]

Hi, so I didn’t really want this to be this big, but one thing led to another…

It started with wanting to fix reconnection and message catchup on Safari iOS. But then if you come back to the tab and messages catch up, losing your scroll position is quite annoying, so I added scroll position saving. And then the hooks started to look a bit dodgy so did a bit of a refactor.

It all seems to work on my two instances - hope it makes sense. Won’t be offended if you reject / chop it into bits / whatever.

Thanks!

—-

Server: when an SSE stream reconnects with last_sequence_id, query ListMessagesSince to backfill messages written between disconnect and reconnect. Previously these were silently lost, causing invisible gaps. On resume, omit context_window_size (only partial messages available).

Client SSE reconnection: on iOS Safari, backgrounding a tab kills TCP connections but EventSource.readyState still reads OPEN. Track when the page was hidden and force-reconnect if backgrounded for >5s. Suppress auto-scroll during catch-up so the user's scroll position is preserved.

Client scroll position: save to localStorage on scroll settle (100ms debounce) and on visibilitychange/beforeunload. Restore via useLayoutEffect after messages render. Use ResizeObserver on .messages-list to re-scroll to bottom when content expands (images loading, tool outputs rendering) during streaming.

Refactor: replace function-in-ref patterns (reconnectRef, forceReconnectRef, handleManualReconnect) with useCallback hooks with proper dependency arrays. Wrap setupMessageStream in useCallback; nest resetHeartbeatTimeout inside it.

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