fix(layers): make restart repair passive and always-on

frontend/src/lib/layerPreferences.ts

@@ -94,10 +94,8 @@ export function getDefaultActiveLayers(): ActiveLayers {
 
 const ACTIVE_LAYER_KEYS = Object.keys(getDefaultActiveLayers()) as (keyof ActiveLayers)[];
 const LAYER_BACKEND_SYNC_INTERVAL_MS = 5000;
-let layerBackendSyncTimer: ReturnType<typeof setInterval> | null = null;
+let layerBackendSyncTimer: number | null = null;
 let layerBackendSyncInFlight = false;
-let layerBackendSeenSuccess = false;
-let layerBackendSawFailure = false;
 let repairedDefaultBackendState = false;
 
 function isBooleanRecord(value: unknown): value is Record<string, boolean> {
@@ -199,10 +197,9 @@ function emitLayerPreferencesChanged(layers: ActiveLayers): void {
  * The dashboard already saves the operator's layer choices locally. The backend,
  * however, resets its fetcher gates on process restart. This reconciliation loop
  * observes the existing GET /api/layers endpoint and replays the saved operator
- * state only when a connection is first established, after an observed outage,
- * or when the backend has unmistakably fallen back to its shipped defaults.
- * It does not continuously overwrite arbitrary non-default server state, so two
- * independent clients do not get into a five-second preference tug-of-war.
+ * state only when the backend is unmistakably back at its shipped defaults.
+ * It deliberately leaves other non-default server state alone, so independent
+ * clients do not get into a five-second preference tug-of-war.
  */
 export async function reconcileActiveLayersWithBackend(): Promise<'synced' | 'noop' | 'offline'> {
   if (typeof window === 'undefined' || typeof fetch !== 'function') return 'offline';
@@ -228,11 +225,7 @@ export async function reconcileActiveLayersWithBackend(): Promise<'synced' | 'no
     const backend = mergeActiveLayers(defaults, rawLayers);
     const mismatch = !layerMapsEqual(desired, backend);
     const backendAtDefaults = layerMapsEqual(backend, defaults);
-    const shouldRepair =
-      mismatch &&
-      (!layerBackendSeenSuccess ||
-        layerBackendSawFailure ||
-        (backendAtDefaults && !repairedDefaultBackendState));
+    const shouldRepair = mismatch && backendAtDefaults && !repairedDefaultBackendState;
 
     if (shouldRepair) {
       const syncResponse = await fetch(`${API_BASE}/api/layers`, {
@@ -245,18 +238,15 @@ export async function reconcileActiveLayersWithBackend(): Promise<'synced' | 'no
       }
       // useDataPolling already listens for this event and refreshes gated data immediately.
       window.dispatchEvent(new Event('sb:layer-toggle'));
-      repairedDefaultBackendState = backendAtDefaults;
-      layerBackendSeenSuccess = true;
-      layerBackendSawFailure = false;
+      repairedDefaultBackendState = true;
       return 'synced';
     }
 
+    // Once the repaired non-default state is observable, arm the watchdog for a
+    // future backend restart that returns the process-local gate to defaults.
     if (!backendAtDefaults) repairedDefaultBackendState = false;
-    layerBackendSeenSuccess = true;
-    layerBackendSawFailure = false;
     return 'noop';
   } catch {
-    layerBackendSawFailure = true;
     return 'offline';
   } finally {
     layerBackendSyncInFlight = false;
@@ -352,3 +342,7 @@ export function loadLayerSectionExpanded(): Record<string, boolean> {
 export function saveLayerSectionExpanded(expanded: Record<string, boolean>): void {
   writeDashboardPrefs({ layerSectionsExpanded: expanded });
 }
+
+// Start the passive restart watchdog as soon as the preferences module is loaded
+// in a real browser. SSR and Vitest are guarded inside ensureLayerBackendSync().
+ensureLayerBackendSync();