CmdPal: Ensure DockWindow property cleans up after itself (#46303)

## Summary of the Pull Request

This PR improves cleanup of DockWindow after itself (since it can be
created and destroyed multiple times during app lifetime).

- Disposes its ViewModel (which it creates).
- Unregisters itself explicitly from WeakReferenceMessenger.
- Ensures that ShellPage closes the dock window when disposed and can't
spawn more.

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## PR Checklist

- [x] Closes: #46302
<!-- - [ ] 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

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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

Co-authored-by: Zach Teutsch <88554871+zateutsch@users.noreply.github.com>

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

@@ -5,7 +5,6 @@
 using System.Runtime.InteropServices;
 using CommunityToolkit.Mvvm.Messaging;
 using ManagedCommon;
-using Microsoft.CmdPal.UI.Helpers;
 using Microsoft.CmdPal.UI.ViewModels;
 using Microsoft.CmdPal.UI.ViewModels.Dock;
 using Microsoft.CmdPal.UI.ViewModels.Messages;
@@ -27,7 +26,6 @@ using Windows.Win32.UI.WindowsAndMessaging;
 using WinRT;
 using WinRT.Interop;
 using WinUIEx;
-using WindowExtensions = Microsoft.CmdPal.UI.Helpers.WindowExtensions;
 
 namespace Microsoft.CmdPal.UI.Dock;
 
@@ -62,6 +60,7 @@ public sealed partial class DockWindow : WindowEx,
     private bool _isUpdatingBackdrop;
     private BackdropParameters? _lastAppliedAcrylicBackdrop;
     private DockSize _lastSize;
+    private bool _isDisposed;
 
     // Store the original WndProc
     private WNDPROC? _originalWndProc;
@@ -671,18 +670,38 @@ public sealed partial class DockWindow : WindowEx,
 
     public void Dispose()
     {
-        DisposeAcrylic();
-        _windowViewModel.Dispose();
+        Cleanup();
+        GC.SuppressFinalize(this);
     }
 
     private void DockWindow_Closed(object sender, WindowEventArgs args)
     {
-        _settingsService.SettingsChanged -= SettingsChangedHandler;
+        Dispose();
+    }
+
+    private void Cleanup()
+    {
+        if (_isDisposed)
+        {
+            return;
+        }
+
+        _isDisposed = true;
+
+        _settingsService?.SettingsChanged -= SettingsChangedHandler;
+
+        Activated -= DockWindow_Activated;
         _themeService.ThemeChanged -= ThemeService_ThemeChanged;
+        WeakReferenceMessenger.Default.UnregisterAll(this);
+
         DisposeAcrylic();
+        _windowViewModel.Dispose();
 
         // Remove our app bar registration
-        DestroyAppBar(_hwnd);
+        if (_appBarData.hWnd != IntPtr.Zero)
+        {
+            DestroyAppBar(_hwnd);
+        }
 
         // Unhook the window procedure
         ShowDesktop.RemoveHook();

src/modules/cmdpal/Microsoft.CmdPal.UI/Pages/ShellPage.xaml.cs

@@ -73,6 +73,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
 
     private CancellationTokenSource? _focusAfterLoadedCts;
     private WeakReference<Page>? _lastNavigatedPageRef;
+    private bool _isDisposed;
 
     public ShellViewModel ViewModel { get; private set; } = App.Current.Services.GetService<ShellViewModel>()!;
 
@@ -308,6 +309,11 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
     {
         _ = DispatcherQueue.TryEnqueue(() =>
         {
+            if (_isDisposed)
+            {
+                return;
+            }
+
             OpenSettings(message.SettingsPageTag);
         });
     }
@@ -528,6 +534,11 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
     {
         _ = DispatcherQueue.TryEnqueue(() =>
         {
+            if (_isDisposed)
+            {
+                return;
+            }
+
             if (message.ShowDock)
             {
                 if (_dockWindow is null)
@@ -829,10 +840,33 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
 
     public void Dispose()
     {
+        if (_isDisposed)
+        {
+            return;
+        }
+
+        _isDisposed = true;
+        WeakReferenceMessenger.Default.UnregisterAll(this);
+
         _focusAfterLoadedCts?.Cancel();
         _focusAfterLoadedCts?.Dispose();
         _focusAfterLoadedCts = null;
 
-        _dockWindow?.Dispose();
+        var dockWindow = _dockWindow;
+        _dockWindow = null;
+
+        if (dockWindow is not null)
+        {
+            if (DispatcherQueue.HasThreadAccess)
+            {
+                dockWindow.Close();
+            }
+            else
+            {
+                DispatcherQueue.TryEnqueue(dockWindow.Close);
+            }
+        }
+
+        GC.SuppressFinalize(this);
     }
 }