feat: the browser terminal on a WebSocket too, long polls when it does not connect or drops

internal/server/hub.go

@@ -324,10 +324,18 @@ func (s *Session) Out() <-chan proto.OutMessage { return s.out }
 
 // PollOut drains queued upstream messages, blocking up to timeout for the first.
 func (s *Session) PollOut(timeout time.Duration) ([]proto.OutMessage, bool) {
+	return s.PollOutUntil(nil, timeout)
+}
+
+// PollOutUntil is PollOut that also gives up when done closes, taking
+// nothing: for a reader that has gone, so no output is lost on it.
+func (s *Session) PollOutUntil(done <-chan struct{}, timeout time.Duration) ([]proto.OutMessage, bool) {
 	var batch []proto.OutMessage
 	select {
 	case m := <-s.out:
 		batch = append(batch, m)
+	case <-done:
+		return nil, true
 	case <-time.After(timeout):
 		return nil, true
 	case <-s.closed: