fix(centralui): Topology live tick behind PollGate, refresh no longer unmounts tree (review 2026-10-21 07/P-2)

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Deployment/Topology.razor

@@ -141,6 +141,7 @@
     private readonly HashSet<int> _loadedSites = new();
     private bool _branchLoading;
     private readonly SemaphoreSlim _loadGate = new(1, 1);
+    private readonly ZB.MOM.WW.ScadaBridge.CentralUI.Services.PollGate _pollGate = new();
     private List<Area> _allAreas = new();
     private Dictionary<int, bool> _stalenessMap = new();
 
@@ -242,25 +243,23 @@
     private void OnLiveUpdatesTick(object? state)
     {
         if (_disposed) return;
-        InvokeAsync(async () =>
-        {
-            if (_disposed || !_liveUpdates) return;
-            try
+        // PollGate (arch-review S3): skip the tick while the previous refresh is still in
+        // flight, so a slow configuration DB cannot stack queries and renderer continuations.
+        _ = _pollGate.RunAsync(
+            () => InvokeAsync(async () =>
             {
+                if (_disposed || !_liveUpdates) return;
                 // Staleness is deliberately OFF the live loop: it only changes when a
                 // template/instance is edited or redeployed, and recomputing it (a
                 // full re-flatten per deployed instance) every 15s is the page's
                 // dominant cost. The poll refreshes only the cheap deployed state.
-                await LoadDeployedStateAsync();
+                await LoadDeployedStateAsync(isRefresh: true);
                 if (_disposed) return;
                 StateHasChanged();
-            }
-            catch (ObjectDisposedException)
-            {
-                // Component disposed between the guard and the dispatch — ignore
-                // (mirrors DebugView's SafeInvokeAsync / CentralUI-009).
-            }
-        });
+            }),
+            // Disposed between the guard and the dispatch (mirrors DebugView's
+            // SafeInvokeAsync / CentralUI-009); LoadDeployedStateAsync reports its own failures.
+            _ => { });
     }
 
     private void OnLiveUpdatesToggled(ChangeEventArgs e)
@@ -339,12 +338,15 @@
     /// This is the live-update timer path; it does not recompute staleness. Internal so the live-update regression
     /// test can drive the timer's code path directly.
     /// </summary>
-    internal async Task LoadDeployedStateAsync()
+    internal async Task LoadDeployedStateAsync(bool isRefresh = false)
     {
         await _loadGate.WaitAsync();
         try
         {
-            _loading = true;
+            // Only the initial/explicit load unmounts the tree behind the spinner; the 15s
+            // live tick (isRefresh) must not, or any render landing mid-tick swaps the whole
+            // tree for the spinner and back (review 2026-10-21 07/P-2).
+            if (!isRefresh) _loading = true;
             _errorMessage = null;
             try
             {
@@ -355,12 +357,16 @@
                 _loadedSites.IntersectWith(permittedSiteIds);
                 foreach (var key in _loadedPages.Keys.Where(k => !permittedSiteIds.Contains(k.Site)).ToArray())
                     _loadedPages.Remove(key);
-                _allInstances.Clear();
-                _allAreas.Clear();
-                foreach (var siteId in _loadedSites)
-                    _allAreas.AddRange(await TemplateEngineRepository.GetAreasBySiteIdAsync(siteId));
-                foreach (var (key, offset) in _loadedPages)
-                    _allInstances.AddRange(await TemplateEngineRepository.GetTopologyInstancesAsync(key.Site, key.Area, offset, InstancePageSize));
+                // Gather into locals and swap, so a render landing mid-fetch never sees
+                // the cleared lists.
+                var areas = new List<Area>();
+                var instances = new List<Instance>();
+                foreach (var siteId in _loadedSites.ToArray())
+                    areas.AddRange(await TemplateEngineRepository.GetAreasBySiteIdAsync(siteId));
+                foreach (var (key, offset) in _loadedPages.ToArray())
+                    instances.AddRange(await TemplateEngineRepository.GetTopologyInstancesAsync(key.Site, key.Area, offset, InstancePageSize));
+                _allAreas = areas;
+                _allInstances = instances;
 
                 BuildTree();
             }