Fix comment inaccuracy in ThemeServiceOnThemeChanged deadlock explanation

Agent-Logs-Url: https://github.com/microsoft/PowerToys/sessions/510eb74c-6f60-41ef-b74f-67b1ed610be6

Co-authored-by: MuyuanMS <116717757+MuyuanMS@users.noreply.github.com>

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/AppearanceSettingsViewModel.cs

@@ -465,7 +465,20 @@ public sealed partial class AppearanceSettingsViewModel : ObservableObject, IDis
 
     private void ThemeServiceOnThemeChanged(object? sender, ThemeChangedEventArgs e)
     {
-        _saveTimer.Debounce(Reapply, TimeSpan.FromMilliseconds(200));
+        // Call Reapply directly rather than via _saveTimer.Debounce.
+        // ThemeService already debounces internally, so ThemeChanged is already
+        // de-duplicated by the time this handler runs. Using Debounce here can
+        // cause a deadlock: a prior Debounce call's DispatcherQueueTimer may still
+        // be trying to unsubscribe from its Tick event, which acquires a WinRT
+        // EventSourceCache write lock. While waiting for that write lock (blocked
+        // by background threads holding read locks during extension-event subscription),
+        // WinRT STA reentrancy lets a nested timer callback reach this handler again.
+        // The inner Debounce call then tries to subscribe to _saveTimer.Tick (a read
+        // lock), but ReaderWriterLockSlim blocks new readers while a writer is pending,
+        // creating a circular wait: the pending writer waits for background readers to
+        // finish, and those background threads wait for the STA to process a COM call
+        // that is itself blocked by the deadlocked nested callback.
+        Reapply();
     }
 
     private void DebouncedReapply()

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/DockAppearanceSettingsViewModel.cs

@@ -295,7 +295,20 @@ public sealed partial class DockAppearanceSettingsViewModel : ObservableObject,
 
     private void ThemeServiceOnThemeChanged(object? sender, ThemeChangedEventArgs e)
     {
-        _saveTimer.Debounce(Reapply, TimeSpan.FromMilliseconds(200));
+        // Call Reapply directly rather than via _saveTimer.Debounce.
+        // ThemeService already debounces internally, so ThemeChanged is already
+        // de-duplicated by the time this handler runs. Using Debounce here can
+        // cause a deadlock: a prior Debounce call's DispatcherQueueTimer may still
+        // be trying to unsubscribe from its Tick event, which acquires a WinRT
+        // EventSourceCache write lock. While waiting for that write lock (blocked
+        // by background threads holding read locks during extension-event subscription),
+        // WinRT STA reentrancy lets a nested timer callback reach this handler again.
+        // The inner Debounce call then tries to subscribe to _saveTimer.Tick (a read
+        // lock), but ReaderWriterLockSlim blocks new readers while a writer is pending,
+        // creating a circular wait: the pending writer waits for background readers to
+        // finish, and those background threads wait for the STA to process a COM call
+        // that is itself blocked by the deadlocked nested callback.
+        Reapply();
     }
 
     private void DebouncedReapply()