Web UI freezes mid-session when the event stream dies silently; only a page refresh recovers #39352

Description

Very often the web UI gets stuck mid-session: the spinner keeps spinning on the last line (e.g. a tool header like "Thinking WHOIS:") but nothing new ever arrives. Refreshing the page shows the run actually finished long ago — I've waited 20 minutes on a session that had completed 19 minutes earlier. Without a refresh it never recovers.

Environment

• opencode 1.18.5, opencode web --port 4096
• Browser on Windows, server inside WSL2 (so the connection crosses the Windows/WSL2 boundary, which is prone to silently killing idle sockets)

Root cause

Two problems stack:

1. The SSE consumer reads the event stream with no liveness check. If the socket dies without a FIN/RST (NAT/proxy eviction, suspend), reader.read() just never resolves — no error, so the reconnect loop in server-sdk.tsx never re-enters. The server's heartbeats can't help: on v2 they're SSE comment frames (: heartbeat), which the parser discards without yielding an event, so the app literally cannot see them.

2. Even when a reconnect does happen, nothing repairs the drift. session_status is only written by session.execution.* events, so a run that finished during the outage stays busy forever, and the active-sessions refetch on server.connected is gated on the query having no data — i.e. it only ever fires on the first connect.

#13947 fixed the missed-events-after-reconnect case, but that only helps when the client notices the disconnect. Here the stream dies silently, so there's no reconnect at all — and the status/timeline drift isn't reconciled either way.

Fix incoming.