Workaround a bug where WindowsXamlHostBase needs to be Disposed when the parent Window is closed.

Directory.Build.props

@@ -18,7 +18,7 @@
     <IsWpfProject Condition="'$(IsDesignProject)' != 'true'">$(MSBuildProjectName.Contains('Wpf'))</IsWpfProject>
     <IsFormsProject Condition="'$(IsDesignProject)' != 'true'">$(MSBuildProjectName.Contains('Forms'))</IsFormsProject>
     <IsSampleProject>$(MSBuildProjectName.Contains('Sample'))</IsSampleProject>
-    <DefaultTargetPlatformVersion>18351</DefaultTargetPlatformVersion>
+    <DefaultTargetPlatformVersion>18362</DefaultTargetPlatformVersion>
     <DefaultTargetPlatformMinVersion>18226</DefaultTargetPlatformMinVersion>
     <PackageOutputPath>$(MSBuildThisFileDirectory)bin\nupkg</PackageOutputPath>
   </PropertyGroup>
@@ -45,7 +45,7 @@
         This also needs to be installed on your local machine. Can do this with PowerShell:
           ./build/Install-WindowsSDKISO.ps1 18351
         -->
-        <TargetPlatformVersion>10.0.18351.0</TargetPlatformVersion>
+        <TargetPlatformVersion>10.0.18362.0</TargetPlatformVersion>
         <!-- XAML Islands require SDK 18226 -->
         <TargetPlatformMinVersion>10.0.18226.0</TargetPlatformMinVersion>
 

Microsoft.Toolkit.Forms.UI.XamlHost/WindowsXamlHostBase.cs

@@ -280,5 +280,15 @@ namespace Microsoft.Toolkit.Forms.UI.XamlHost
                 UpdateDpiScalingFactor();
             }
         }
+
+        /// <summary>
+        /// Override that disposes the current instance when the parent handle has been destroyed
+        /// </summary>
+        /// <param name="e">Ignored</param>
+        protected override void OnHandleDestroyed(EventArgs e)
+        {
+            this.Dispose();
+            base.OnHandleDestroyed(e);
+        }
     }
 }

Microsoft.Toolkit.Wpf.UI.XamlHost/WindowsXamlHostBase.cs

@@ -192,6 +192,8 @@ namespace Microsoft.Toolkit.Wpf.UI.XamlHost
         /// </summary>
         public bool IsDisposed { get; private set; }
 
+        private System.Windows.Window _parentWindow;
+
         /// <summary>
         /// Creates <see cref="WUX.Application" /> object, wrapped <see cref="WUX.Hosting.DesktopWindowXamlSource" /> instance; creates and
         /// sets root UWP XAML element on <see cref="WUX.Hosting.DesktopWindowXamlSource" />.
@@ -200,6 +202,12 @@ namespace Microsoft.Toolkit.Wpf.UI.XamlHost
         /// <returns>Handle to XAML window</returns>
         protected override HandleRef BuildWindowCore(HandleRef hwndParent)
         {
+            this._parentWindow = System.Windows.Window.GetWindow(this);
+            if (_parentWindow != null)
+            {
+                _parentWindow.Closed += this.OnParentClosed;
+            }
+
             ComponentDispatcher.ThreadFilterMessage += this.OnThreadFilterMessage;
 
             // 'EnableMouseInPointer' is called by the WindowsXamlManager during initialization. No need
@@ -230,6 +238,16 @@ namespace Microsoft.Toolkit.Wpf.UI.XamlHost
             }
         }
 
+        /// <summary>
+        /// Disposes the current instance in response to the parent window getting destroyed.
+        /// </summary>
+        /// <param name="sender">Paramter sender is ignored</param>
+        /// <param name="e">Parameter args is ignored</param>
+        private void OnParentClosed(object sender, EventArgs e)
+        {
+            this.Dispose();
+        }
+
         /// <summary>
         /// WPF framework request to destroy control window.  Cleans up the HwndIslandSite created by DesktopWindowXamlSource
         /// </summary>
@@ -260,6 +278,12 @@ namespace Microsoft.Toolkit.Wpf.UI.XamlHost
                 {
                     _xamlSource.TakeFocusRequested -= OnTakeFocusRequested;
                 }
+
+                if (_parentWindow != null)
+                {
+                    _parentWindow.Closed -= this.OnParentClosed;
+                    _parentWindow = null;
+                }
             }
 
             // Free any unmanaged objects here.