Fix #916 — chart tooltips break after tab switch (real root cause)

Dashboard/Controls/MemoryContent.xaml.cs

@@ -106,11 +106,10 @@ public MemoryContent()
             SetupChartContextMenus();
             Loaded += OnLoaded;
             Helpers.ThemeManager.ThemeChanged += OnThemeChanged;
-            Unloaded += (_, _) =>
-            {
-                Helpers.ThemeManager.ThemeChanged -= OnThemeChanged;
-                DisposeChartHelpers();
-            };
+            /* WPF fires Unloaded on every TabControl tab switch, not just on destruction.
+               Tearing down chart hover helpers here unsubscribes their MouseMove handlers
+               and they are never re-registered when the user returns — this is the
+               root cause of #916. Final disposal happens via ServerTab.CleanupOnClose. */
 
             // Apply dark theme immediately so charts don't flash white before data loads
             TabHelpers.ApplyThemeToChart(MemoryStatsOverviewChart);
@@ -136,6 +135,7 @@ public void DisposeChartHelpers()
             _memoryClerksHover?.Dispose();
             _planCacheHover?.Dispose();
             _memoryPressureEventsHover?.Dispose();
+            Helpers.ThemeManager.ThemeChanged -= OnThemeChanged;
         }
 
         private void OnLoaded(object sender, RoutedEventArgs e)

Dashboard/Controls/QueryPerformanceContent.xaml.cs

@@ -241,8 +241,10 @@ private void OnUnloaded(object sender, RoutedEventArgs e)
             _qsRegressionsUnfilteredData = null;
             _lrqPatternsUnfilteredData = null;
 
-            DisposeChartHelpers();
-            Helpers.ThemeManager.ThemeChanged -= OnThemeChanged;
+            /* WPF fires Unloaded on every TabControl tab switch, not just on destruction.
+               Tearing down chart hover helpers or unsubscribing ThemeManager here breaks
+               tooltips and theme refresh after a tab switch (#916). Final cleanup happens
+               via ServerTab.CleanupOnClose → DisposeChartHelpers. */
         }
 
         public void DisposeChartHelpers()
@@ -251,6 +253,7 @@ public void DisposeChartHelpers()
             _procDurationHover?.Dispose();
             _qsDurationHover?.Dispose();
             _execTrendsHover?.Dispose();
+            Helpers.ThemeManager.ThemeChanged -= OnThemeChanged;
         }
 
         private void OnThemeChanged(string _)

Dashboard/Controls/ResourceMetricsContent.xaml.cs

@@ -130,11 +130,10 @@ public ResourceMetricsContent()
             SetupChartContextMenus();
             Loaded += OnLoaded;
             Helpers.ThemeManager.ThemeChanged += OnThemeChanged;
-            Unloaded += (_, _) =>
-            {
-                Helpers.ThemeManager.ThemeChanged -= OnThemeChanged;
-                DisposeChartHelpers();
-            };
+            /* WPF fires Unloaded on every TabControl tab switch, not just on destruction.
+               Tearing down chart hover helpers here unsubscribes their MouseMove handlers
+               and they are never re-registered when the user returns — this is the
+               root cause of #916. Final disposal happens via ServerTab.CleanupOnClose. */
 
             // Apply dark theme immediately so charts don't flash white before data loads
             TabHelpers.ApplyThemeToChart(LatchStatsChart);
@@ -175,6 +174,7 @@ public void DisposeChartHelpers()
             _waitStatsHover?.Dispose();
             _tempdbStatsHover?.Dispose();
             _tempDbLatencyHover?.Dispose();
+            Helpers.ThemeManager.ThemeChanged -= OnThemeChanged;
         }
 
         private void OnLoaded(object sender, RoutedEventArgs e)

Dashboard/Controls/SystemEventsContent.xaml.cs

@@ -175,20 +175,40 @@ public SystemEventsContent()
 
         private void OnUnloaded(object sender, RoutedEventArgs e)
         {
-            /* Unsubscribe from filter popup events to prevent memory leaks */
+            /* WPF fires Unloaded on every TabControl tab switch, not just on destruction.
+               Unsubscribing ThemeManager or filter-popup events here breaks them on
+               return to the tab (#916 family). Final cleanup happens via
+               ServerTab.CleanupOnClose → DisposeChartHelpers. */
+        }
+
+        public void DisposeChartHelpers()
+        {
+            _badPagesHover?.Dispose();
+            _dumpRequestsHover?.Dispose();
+            _accessViolationsHover?.Dispose();
+            _writeAccessViolationsHover?.Dispose();
+            _nonYieldingTasksHover?.Dispose();
+            _latchWarningsHover?.Dispose();
+            _sickSpinlocksHover?.Dispose();
+            _cpuComparisonHover?.Dispose();
+            _severeErrorsHover?.Dispose();
+            _ioIssuesHover?.Dispose();
+            _longestPendingIoHover?.Dispose();
+            _schedulerIssuesHover?.Dispose();
+            _memoryConditionsHover?.Dispose();
+            _cpuTasksHover?.Dispose();
+            _memoryBrokerHover?.Dispose();
+            _memoryBrokerRatioHover?.Dispose();
+            _memoryNodeOomHover?.Dispose();
+            _memoryNodeOomUtilHover?.Dispose();
+            _memoryNodeOomMemoryHover?.Dispose();
+
             if (_filterPopupContent != null)
             {
                 _filterPopupContent.FilterApplied -= FilterPopup_FilterApplied;
                 _filterPopupContent.FilterCleared -= FilterPopup_FilterCleared;
             }
 
-            /* Clear large data collections to free memory */
-            _systemHealthUnfilteredData = null;
-            _severeErrorsUnfilteredData = null;
-            _ioIssuesUnfilteredData = null;
-            _memoryBrokerUnfilteredData = null;
-            _memoryNodeOOMUnfilteredData = null;
-
             Helpers.ThemeManager.ThemeChanged -= OnThemeChanged;
         }
 

Dashboard/ServerTab.xaml.cs

@@ -275,6 +275,7 @@ public void DisposeChartHelpers()
             MemoryTab.DisposeChartHelpers();
             ResourceMetricsContent.DisposeChartHelpers();
             PerformanceTab.DisposeChartHelpers();
+            SystemEventsContent.DisposeChartHelpers();
         }
 
         public void RefreshAutoRefreshSettings()