Live wall review fixes: no not-found flash or map whiplash when the selected incident resolves (#239)

useIncident reports loading until its answer belongs to the current id
(the render before the effect read the previous id's loading=false), and
clears a stale doc on a denied read. LiveView holds the last selected doc
for the same id while the by-id lookup is pending, so a watched incident
resolving no longer deselects (fly to region) and reselects (fly back).

Follow-up for the panel's 'active'/elapsed copy on resolved incidents: #275.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-frontend/lib/useIncidents.ts

@@ -86,17 +86,19 @@ export function useIncidents(limitCount = 100, dateFrom?: Date, dateTo?: Date) {
 export function useIncident(incidentId: string | null) {
   const [incident, setIncident] = useState<IncidentRecord | null>(null);
   const [loading, setLoading] = useState(true);
+  // The id the current `incident`/`loading` answer belongs to. On the render
+  // where the id changes the effect hasn't run yet, so `loading` alone would
+  // still read false from the previous id and a caller would flash "not found".
+  const [settledId, setSettledId] = useState<string | null>(null);
 
   useEffect(() => {
     if (!incidentId) { setLoading(false); return; }
-    // A new id is a new lookup: without this, `loading` stays false from the
-    // previous id and a caller reads "not found" until the snapshot lands.
     setLoading(true);
     let unsubFirestore: (() => void) | undefined;
 
     const unsubAuth = onAuthStateChanged(auth, (user) => {
       if (unsubFirestore) { unsubFirestore(); unsubFirestore = undefined; }
-      if (!user) { setLoading(false); return; }
+      if (!user) { setLoading(false); setSettledId(incidentId); return; }
 
       const ref = doc(db, "incidents", incidentId);
       unsubFirestore = onSnapshot(ref, (snap) => {
@@ -111,16 +113,19 @@ export function useIncident(incidentId: string | null) {
           setIncident(null);
         }
         setLoading(false);
+        setSettledId(incidentId);
       }, (err: FirestoreError) => {
         console.error("useIncident:", err);
+        setIncident(null); // a denied/failed read must not leave the previous id's doc
         setLoading(false);
+        setSettledId(incidentId);
       });
     });
 
     return () => { unsubAuth(); if (unsubFirestore) unsubFirestore(); };
   }, [incidentId]);
 
-  return { incident, loading };
+  return { incident, loading: loading || (!!incidentId && settledId !== incidentId) };
 }
 
 export function useActiveIncidents() {