CmdPal: Transparent window (#45159)

## Summary

This PR adds:
- Backdrop material customization
  - Alongside acrylic, the following options are now available:
  - Transparent background
  - Mica background
- Background material opacity
  - Lets you control how transparent the background is

## Pictures? Pictures!

<img width="1491" height="928" alt="image"
src="https://github.com/user-attachments/assets/ff4e9e06-fcf1-4f05-bc0a-fb70dc4f39be"
/>



https://github.com/user-attachments/assets/84e83279-afab-481e-b904-f054318c5d2f

<img width="977" height="628" alt="image"
src="https://github.com/user-attachments/assets/241a228d-af3f-448a-94a6-0a282218bd8c"
/>


## PR Checklist

- [x] Closes: #44197
<!-- - [ ] 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
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/MainWindow.xaml.cs

@@ -31,6 +31,7 @@ using Windows.ApplicationModel.Activation;
 using Windows.Foundation;
 using Windows.Graphics;
 using Windows.System;
+using Windows.UI;
 using Windows.Win32;
 using Windows.Win32.Foundation;
 using Windows.Win32.Graphics.Dwm;
@@ -80,7 +81,9 @@ public sealed partial class MainWindow : WindowEx,
     private int _sessionErrorCount;
 
     private DesktopAcrylicController? _acrylicController;
+    private MicaController? _micaController;
     private SystemBackdropConfiguration? _configurationSource;
+    private bool _isUpdatingBackdrop;
     private TimeSpan _autoGoHomeInterval = Timeout.InfiniteTimeSpan;
 
     private WindowPosition _currentWindowPosition = new();
@@ -109,7 +112,7 @@ public sealed partial class MainWindow : WindowEx,
             CommandPaletteHost.SetHostHwnd((ulong)_hwnd.Value);
         }
 
-        SetAcrylic();
+        InitializeBackdropSupport();
 
         _hiddenOwnerBehavior.ShowInTaskbar(this, Debugger.IsAttached);
 
@@ -158,7 +161,7 @@ public sealed partial class MainWindow : WindowEx,
         App.Current.Services.GetService<SettingsModel>()!.SettingsChanged += SettingsChangedHandler;
 
         // Make sure that we update the acrylic theme when the OS theme changes
-        RootElement.ActualThemeChanged += (s, e) => DispatcherQueue.TryEnqueue(UpdateAcrylic);
+        RootElement.ActualThemeChanged += (s, e) => DispatcherQueue.TryEnqueue(UpdateBackdrop);
 
         // Hardcoding event name to avoid bringing in the PowerToys.interop dependency. Event name must match CMDPAL_SHOW_EVENT from shared_constants.h
         NativeEventWaiter.WaitForEventLoop("Local\\PowerToysCmdPal-ShowEvent-62336fcd-8611-4023-9b30-091a6af4cc5a", () =>
@@ -185,7 +188,7 @@ public sealed partial class MainWindow : WindowEx,
 
     private void ThemeServiceOnThemeChanged(object? sender, ThemeChangedEventArgs e)
     {
-        UpdateAcrylic();
+        UpdateBackdrop();
     }
 
     private static void LocalKeyboardListener_OnKeyPressed(object? sender, LocalKeyboardListenerKeyPressedEventArgs e)
@@ -280,48 +283,170 @@ public sealed partial class MainWindow : WindowEx,
         _autoGoHomeTimer.Interval = _autoGoHomeInterval;
     }
 
-    private void SetAcrylic()
+    private void InitializeBackdropSupport()
     {
-        if (DesktopAcrylicController.IsSupported())
+        if (DesktopAcrylicController.IsSupported() || MicaController.IsSupported())
         {
-            // Hooking up the policy object.
             _configurationSource = new SystemBackdropConfiguration
             {
-                // Initial configuration state.
                 IsInputActive = true,
             };
-            UpdateAcrylic();
         }
     }
 
-    private void UpdateAcrylic()
+    private void UpdateBackdrop()
     {
+        // Prevent re-entrance when backdrop changes trigger ActualThemeChanged
+        if (_isUpdatingBackdrop)
+        {
+            return;
+        }
+
+        _isUpdatingBackdrop = true;
+
+        var backdrop = _themeService.Current.BackdropParameters;
+        var isImageMode = ViewModel.ShowBackgroundImage;
+        var config = BackdropStyles.Get(backdrop.Style);
+
         try
         {
-            if (_acrylicController != null)
+            switch (config.ControllerKind)
             {
-                _acrylicController.RemoveAllSystemBackdropTargets();
-                _acrylicController.Dispose();
+                case BackdropControllerKind.Solid:
+                    CleanupBackdropControllers();
+                    var tintColor = Color.FromArgb(
+                        (byte)(backdrop.EffectiveOpacity * 255),
+                        backdrop.TintColor.R,
+                        backdrop.TintColor.G,
+                        backdrop.TintColor.B);
+                    SetupTransparentBackdrop(tintColor);
+                    break;
+
+                case BackdropControllerKind.Mica:
+                case BackdropControllerKind.MicaAlt:
+                    SetupMica(backdrop, isImageMode, config.ControllerKind);
+                    break;
+
+                case BackdropControllerKind.Acrylic:
+                case BackdropControllerKind.AcrylicThin:
+                default:
+                    SetupDesktopAcrylic(backdrop, isImageMode, config.ControllerKind);
+                    break;
             }
-
-            var backdrop = _themeService.Current.BackdropParameters;
-            _acrylicController = new DesktopAcrylicController
-            {
-                TintColor = backdrop.TintColor,
-                TintOpacity = backdrop.TintOpacity,
-                FallbackColor = backdrop.FallbackColor,
-                LuminosityOpacity = backdrop.LuminosityOpacity,
-            };
-
-            // Enable the system backdrop.
-            // Note: Be sure to have "using WinRT;" to support the Window.As<...>() call.
-            _acrylicController.AddSystemBackdropTarget(this.As<ICompositionSupportsSystemBackdrop>());
-            _acrylicController.SetSystemBackdropConfiguration(_configurationSource);
         }
         catch (Exception ex)
         {
             Logger.LogError("Failed to update backdrop", ex);
         }
+        finally
+        {
+            _isUpdatingBackdrop = false;
+        }
+    }
+
+    private void SetupTransparentBackdrop(Color tintColor)
+    {
+        if (SystemBackdrop is TransparentTintBackdrop existingBackdrop)
+        {
+            existingBackdrop.TintColor = tintColor;
+        }
+        else
+        {
+            SystemBackdrop = new TransparentTintBackdrop { TintColor = tintColor };
+        }
+    }
+
+    private void CleanupBackdropControllers()
+    {
+        if (_acrylicController is not null)
+        {
+            _acrylicController.RemoveAllSystemBackdropTargets();
+            _acrylicController.Dispose();
+            _acrylicController = null;
+        }
+
+        if (_micaController is not null)
+        {
+            _micaController.RemoveAllSystemBackdropTargets();
+            _micaController.Dispose();
+            _micaController = null;
+        }
+    }
+
+    private void SetupDesktopAcrylic(BackdropParameters backdrop, bool isImageMode, BackdropControllerKind kind)
+    {
+        CleanupBackdropControllers();
+
+        // Fall back to solid color if acrylic not supported
+        if (_configurationSource is null || !DesktopAcrylicController.IsSupported())
+        {
+            SetupTransparentBackdrop(backdrop.FallbackColor);
+            return;
+        }
+
+        // DesktopAcrylicController and SystemBackdrop can't be active simultaneously
+        SystemBackdrop = null;
+
+        // Image mode: no tint here, BlurImageControl handles it (avoids double-tinting)
+        var effectiveTintOpacity = isImageMode
+            ? 0.0f
+            : backdrop.EffectiveOpacity;
+
+        _acrylicController = new DesktopAcrylicController
+        {
+            Kind = kind == BackdropControllerKind.AcrylicThin
+                ? DesktopAcrylicKind.Thin
+                : DesktopAcrylicKind.Default,
+            TintColor = backdrop.TintColor,
+            TintOpacity = effectiveTintOpacity,
+            FallbackColor = backdrop.FallbackColor,
+            LuminosityOpacity = backdrop.EffectiveLuminosityOpacity,
+        };
+
+        // Requires "using WinRT;" for Window.As<>()
+        _acrylicController.AddSystemBackdropTarget(this.As<ICompositionSupportsSystemBackdrop>());
+        _acrylicController.SetSystemBackdropConfiguration(_configurationSource);
+    }
+
+    private void SetupMica(BackdropParameters backdrop, bool isImageMode, BackdropControllerKind kind)
+    {
+        CleanupBackdropControllers();
+
+        // Fall back to solid color if Mica not supported
+        if (_configurationSource is null || !MicaController.IsSupported())
+        {
+            SetupTransparentBackdrop(backdrop.FallbackColor);
+            return;
+        }
+
+        // MicaController and SystemBackdrop can't be active simultaneously
+        SystemBackdrop = null;
+        _configurationSource.Theme = _themeService.Current.Theme == ElementTheme.Dark
+            ? SystemBackdropTheme.Dark
+            : SystemBackdropTheme.Light;
+
+        var hasColorization = _themeService.Current.HasColorization || isImageMode;
+
+        _micaController = new MicaController
+        {
+            Kind = kind == BackdropControllerKind.MicaAlt
+                ? MicaKind.BaseAlt
+                : MicaKind.Base,
+        };
+
+        // Only set tint properties when colorization is active
+        // Otherwise let system handle light/dark theme defaults automatically
+        if (hasColorization)
+        {
+            // Image mode: no tint here, BlurImageControl handles it (avoids double-tinting)
+            _micaController.TintColor = backdrop.TintColor;
+            _micaController.TintOpacity = isImageMode ? 0.0f : backdrop.EffectiveOpacity;
+            _micaController.FallbackColor = backdrop.FallbackColor;
+            _micaController.LuminosityOpacity = backdrop.EffectiveLuminosityOpacity;
+        }
+
+        _micaController.AddSystemBackdropTarget(this.As<ICompositionSupportsSystemBackdrop>());
+        _micaController.SetSystemBackdropConfiguration(_configurationSource);
     }
 
     private void ShowHwnd(IntPtr hwndValue, MonitorBehavior target)
@@ -637,12 +762,8 @@ public sealed partial class MainWindow : WindowEx,
 
     private void DisposeAcrylic()
     {
-        if (_acrylicController is not null)
-        {
-            _acrylicController.Dispose();
-            _acrylicController = null!;
-            _configurationSource = null!;
-        }
+        CleanupBackdropControllers();
+        _configurationSource = null!;
     }
 
     // Updates our window s.t. the top of the window is draggable.