Harden worker federation and operator UI

internal/ui/ui.go

@@ -136,7 +136,9 @@ func (s Server) detail(ctx context.Context, id string) (TaskDetail, error) {
 	if !ok {
 		return TaskDetail{}, domain.ErrNotFound
 	}
-	d := TaskDetail{Task: t, Actions: actions(t)}
+	// Keep collection fields as JSON arrays for browser clients, including
+	// older task records that genuinely have no retained events.
+	d := TaskDetail{Task: t, Events: []domain.Event{}, Actions: actions(t)}
 	for _, e := range s.Store.Events(0) {
 		if e.TaskID != id {
 			continue
@@ -165,9 +167,14 @@ func (s Server) detail(ctx context.Context, id string) (TaskDetail, error) {
 			session.Blocker = "capture unavailable: " + err.Error()
 		}
 		d.Session = session
+	} else if t.State == domain.StateBlocked {
+		// A blocked task has no active lease by definition, but must retain its
+		// last observed pane evidence instead of rendering an unexplained void.
+		d.Session = &Session{PaneID: t.LastPaneID, HarnessID: t.LastHarness, AgentStatus: t.PaneState, Blocker: t.Blocker}
 	}
 	return d, nil
 }
+
 // captureRevision identifies *what the operator saw*, not when they saw it.
 // B20: this was UnixNano, so it changed on every read and said nothing about
 // whether the pane had changed. A content hash changes if and only if the
@@ -206,7 +213,15 @@ func actions(t domain.Task) []Action {
 	return []Action{{ID: "handoff", Enabled: active, Reason: "requires a live leased session"}, {ID: "release", Enabled: active, Needs: []string{"reason or handoff_ref"}}, {ID: "block", Enabled: active, Needs: []string{"blocker"}}, {ID: "complete", Enabled: active, Needs: []string{"report_ref", "receipt"}}}
 }
 func (s Server) Overview(ctx context.Context) Overview {
-	out := Overview{Tasks: s.Store.Tasks(), UpdatedAt: time.Now().UTC()}
+	// JSON null is not an empty collection to browser clients. In particular,
+	// the shell renders the number of active sessions before any page-level
+	// loading state, so a nil Sessions slice made an otherwise healthy empty
+	// worker pool crash the entire SPA on `sessions.length`.
+	tasks := s.Store.Tasks()
+	if tasks == nil {
+		tasks = []domain.Task{}
+	}
+	out := Overview{Tasks: tasks, Workers: []federation.Worker{}, Sessions: []Session{}, UpdatedAt: time.Now().UTC()}
 	if s.Workers != nil {
 		out.Workers = s.Workers.Snapshot()
 	}