CmdPal: Reduce DockWindow backdrop switching and visual artifacts (#46309)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request

This PR reduces "blinking" of dock when (any) CmdPal settings changes.
It handles only backdrop, not icons.

- Avoids recreating the acrylic controller when the effective backdrop
parameters have not changed.
- Reuses the transparent backdrop instead of reassigning it during dock
refreshes.
- Cleans up backdrop controllers only when switching backdrop modes or
disposing the window.
- Removes obsolete dock-specific backdrop helper logic now handled
directly in DockWindow.


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## PR Checklist

- [x] Closes: #46308
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
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for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
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- [ ] **Documentation updated:** If checked, please file a pull request
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and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Dock/DockSettingsToViews.cs

@@ -4,7 +4,6 @@
 
 using Microsoft.CmdPal.UI.ViewModels.Settings;
 using Windows.Win32;
-using WinUIEx;
 
 namespace Microsoft.CmdPal.UI.Dock;
 
@@ -32,16 +31,6 @@ internal static class DockSettingsToViews
         };
     }
 
-    public static Microsoft.UI.Xaml.Media.SystemBackdrop? GetSystemBackdrop(DockBackdrop backdrop)
-    {
-        return backdrop switch
-        {
-            DockBackdrop.Transparent => new TransparentTintBackdrop(),
-            DockBackdrop.Acrylic => null,
-            _ => throw new NotImplementedException(),
-        };
-    }
-
     public static uint GetAppBarEdge(DockSide side)
     {
         return side switch

src/modules/cmdpal/Microsoft.CmdPal.UI/Dock/DockWindow.xaml.cs

@@ -59,6 +59,8 @@ public sealed partial class DockWindow : WindowEx,
     private DockControl _dock;
     private DesktopAcrylicController? _acrylicController;
     private SystemBackdropConfiguration? _configurationSource;
+    private bool _isUpdatingBackdrop;
+    private BackdropParameters? _lastAppliedAcrylicBackdrop;
     private DockSize _lastSize;
 
     // Store the original WndProc
@@ -78,6 +80,7 @@ public sealed partial class DockWindow : WindowEx,
         viewModel = serviceProvider.GetService<DockViewModel>()!;
         _themeService = serviceProvider.GetRequiredService<IThemeService>();
         _themeService.ThemeChanged += ThemeService_ThemeChanged;
+        InitializeBackdropSupport();
         _windowViewModel = new DockWindowViewModel(_themeService);
         _dock = new DockControl(viewModel);
 
@@ -156,14 +159,7 @@ public sealed partial class DockWindow : WindowEx,
     private void UpdateSettingsOnUiThread()
     {
         this.viewModel.UpdateSettings(_settings);
-
-        SystemBackdrop = DockSettingsToViews.GetSystemBackdrop(_settings.Backdrop);
-
-        // If the backdrop is acrylic, things are more complicated
-        if (_settings.Backdrop == DockBackdrop.Acrylic)
-        {
-            SetAcrylic();
-        }
+        UpdateBackdrop();
 
         _dock.UpdateSettings(_settings);
         var side = DockSettingsToViews.GetAppBarEdge(_settings.Side);
@@ -183,31 +179,98 @@ public sealed partial class DockWindow : WindowEx,
         CreateAppBar(_hwnd);
     }
 
-    // We want to use DesktopAcrylicKind.Thin and custom colors as this is the default material
-    // other Shell surfaces are using, this cannot be set in XAML however.
-    private void SetAcrylic()
+    private void InitializeBackdropSupport()
     {
         if (DesktopAcrylicController.IsSupported())
         {
-            // Hooking up the policy object.
             _configurationSource = new SystemBackdropConfiguration
             {
-                // Initial configuration state.
                 IsInputActive = true,
             };
-            UpdateAcrylic();
         }
     }
 
-    private void UpdateAcrylic()
+    private void UpdateBackdrop()
     {
-        if (_acrylicController != null)
+        // Prevent re-entrance when backdrop changes trigger theme refresh work.
+        if (_isUpdatingBackdrop)
+        {
+            return;
+        }
+
+        _isUpdatingBackdrop = true;
+
+        try
+        {
+            switch (_settings.Backdrop)
+            {
+                case DockBackdrop.Transparent:
+                    if (SystemBackdrop is not TransparentTintBackdrop)
+                    {
+                        CleanupBackdropControllers();
+                        SetupTransparentBackdrop();
+                    }
+
+                    break;
+
+                case DockBackdrop.Acrylic:
+                default:
+                    SetupDesktopAcrylic(_themeService.CurrentDockTheme.BackdropParameters);
+                    break;
+            }
+        }
+        catch (Exception ex)
+        {
+            Logger.LogError("Failed to update dock backdrop", ex);
+        }
+        finally
+        {
+            _isUpdatingBackdrop = false;
+        }
+    }
+
+    private void SetupTransparentBackdrop()
+    {
+        if (SystemBackdrop is not TransparentTintBackdrop)
+        {
+            SystemBackdrop = new TransparentTintBackdrop();
+        }
+
+        _lastAppliedAcrylicBackdrop = null;
+    }
+
+    private void CleanupBackdropControllers()
+    {
+        if (_acrylicController is not null)
         {
             _acrylicController.RemoveAllSystemBackdropTargets();
             _acrylicController.Dispose();
+            _acrylicController = null;
         }
 
-        var backdrop = _themeService.CurrentDockTheme.BackdropParameters;
+        _lastAppliedAcrylicBackdrop = null;
+    }
+
+    private void SetupDesktopAcrylic(BackdropParameters backdrop)
+    {
+        var needsAcrylicUpdate = _acrylicController is null || _lastAppliedAcrylicBackdrop != backdrop;
+        if (!needsAcrylicUpdate)
+        {
+            return;
+        }
+
+        CleanupBackdropControllers();
+
+        // Fall back to the transparent backdrop if acrylic is not supported.
+        if (_configurationSource is null || !DesktopAcrylicController.IsSupported())
+        {
+            SetupTransparentBackdrop();
+            return;
+        }
+
+        // DesktopAcrylicController and SystemBackdrop can't be active simultaneously.
+        SystemBackdrop = null;
+
         _acrylicController = new DesktopAcrylicController
         {
             Kind = DesktopAcrylicKind.Thin,
@@ -221,29 +284,20 @@ public sealed partial class DockWindow : WindowEx,
         // Note: Be sure to have "using WinRT;" to support the Window.As<...>() call.
         _acrylicController.AddSystemBackdropTarget(this.As<ICompositionSupportsSystemBackdrop>());
         _acrylicController.SetSystemBackdropConfiguration(_configurationSource);
+        _lastAppliedAcrylicBackdrop = backdrop;
     }
 
     private void DisposeAcrylic()
     {
-        if (_acrylicController is not null)
-        {
-            _acrylicController.Dispose();
-            _acrylicController = null!;
-            _configurationSource = null!;
-        }
+        CleanupBackdropControllers();
+        _configurationSource = null;
     }
 
     private void ThemeService_ThemeChanged(object? sender, ThemeChangedEventArgs e)
     {
         DispatcherQueue.TryEnqueue(() =>
         {
-            // We only need to handle acrylic here.
-            // Transparent background is handled directly in XAML by binding to
-            // the DockWindowViewModel's ColorizationColor properties.
-            if (_settings.Backdrop == DockBackdrop.Acrylic)
-            {
-                UpdateAcrylic();
-            }
+            UpdateBackdrop();
 
             // ActualTheme / RequestedTheme sync,
             // as pilfered from WindowThemeSynchronizer