fix(dashboard): alarm poll loop retries through faults and surfaces them; correct stale session-events empty-state copy

docs/GatewayDashboardDesign.md

@@ -252,6 +252,18 @@ dispatcher, and disposal can run on it. The two are drained concurrently, so the
 bound on disposal is 5 seconds in total rather than per loop — a wedged dispatcher
 blocks both loops at once, and draining them in sequence would time out twice.
 
+Both loops handle faults *inside* the loop and retry: only cancellation ends them.
+A failing alarm query or a render that faults on one tick leaves the page's last
+rows in place and is retried on the next tick — a fault that stopped polling for the
+life of the page would leave stale rows behind with nothing to say so. The poll loop
+also surfaces the fault in the same `Alarm query failed` banner that a query error
+uses, and clears it on the first tick that succeeds; that dispatch is itself
+best-effort, because the fault being reported may be an `InvokeAsync` against a
+disposed renderer, in which case reporting fails the same way and the loop simply
+exits on its next cancellation check. Neither loop method can fault, which is what
+the bounded drain relies on — it must never observe a faulted loop task, since that
+would surface out of `DisposeAsync` and skip the `CancellationTokenSource` dispose.
+
 ### SignalR hubs (remote clients)
 
 Updates for out-of-process clients flow over three SignalR hubs, all guarded by the

src/ZB.MOM.WW.MxGateway.Server/Dashboard/Components/Pages/AlarmsPage.razor

@@ -309,27 +309,76 @@
         };
     }
 
+    // Fault handling sits inside the loop, matching ProviderStatusLoopAsync: a query or render
+    // fault on one tick is transient (a provider blip, a momentarily unavailable session), so it
+    // is surfaced on the page and retried on the next tick rather than ending polling for the
+    // life of the page. Cancellation is the only exit. The loop method itself therefore cannot
+    // fault, which is what DrainAsync in DisposeAsync relies on.
     private async Task PollLoopAsync()
+    {
+        if (!await PollOnceAsync().ConfigureAwait(false))
+        {
+            return;
+        }
+
+        using PeriodicTimer timer = new(TimeSpan.FromSeconds(3));
+        while (true)
+        {
+            try
+            {
+                if (!await timer.WaitForNextTickAsync(_cts.Token).ConfigureAwait(false))
+                {
+                    return;
+                }
+            }
+            catch (OperationCanceledException)
+            {
+                return;
+            }
+
+            if (!await PollOnceAsync().ConfigureAwait(false))
+            {
+                return;
+            }
+        }
+    }
+
+    // Returns false only when cancellation has ended the poll; a non-cancellation fault returns
+    // true so the caller waits for the next tick and tries again.
+    private async Task<bool> PollOnceAsync()
     {
         try
         {
             await InvokeAsync(RefreshAlarmsAsync).ConfigureAwait(false);
-            using PeriodicTimer timer = new(TimeSpan.FromSeconds(3));
-            while (await timer.WaitForNextTickAsync(_cts.Token).ConfigureAwait(false))
-            {
-                await InvokeAsync(RefreshAlarmsAsync).ConfigureAwait(false);
-            }
+            return true;
         }
         catch (OperationCanceledException)
         {
+            return false;
+        }
+        catch (Exception ex)
+        {
+            await ReportPollFaultAsync(ex).ConfigureAwait(false);
+            return true;
+        }
+    }
+
+    private async Task ReportPollFaultAsync(Exception fault)
+    {
+        try
+        {
+            await InvokeAsync(() =>
+            {
+                _queryError = fault.Message;
+                StateHasChanged();
+            }).ConfigureAwait(false);
         }
         catch
         {
-            // Catch-all for the same reason ProviderStatusLoopAsync has one: a teardown race
-            // can fault InvokeAsync (a disposed renderer) after cancellation has already been
-            // requested. Letting that fault the task would surface it out of the drain in
-            // DisposeAsync, skipping _cts.Dispose(). The loop ends here and the page holds its
-            // last rendered rows — it is being disposed or has nothing left to poll with.
+            // Reporting is best-effort: the fault being reported may itself be the teardown race
+            // this catch-all exists for — an InvokeAsync against a disposed renderer — in which
+            // case the dispatch fails the same way and there is no page left to show it on. The
+            // poll loop keeps ticking either way and exits on the next cancellation check.
         }
     }
 

src/ZB.MOM.WW.MxGateway.Server/Dashboard/Components/Pages/SessionDetailsPage.razor

@@ -117,8 +117,9 @@ else
         @if (_recentEvents.Count == 0)
         {
             <div class="empty-state">
-                Waiting for events. The dashboard mirrors the session's gRPC event stream — events
-                appear here only while a gRPC client is also consuming this session's events.
+                Waiting for events. The dashboard subscribes to this session's events directly, so
+                rows appear as the session's worker emits them while this page is open — no gRPC
+                client has to be consuming the session.
             </div>
         }
         else