Fix TopMost handling for Popups on Windows

samples/ControlCatalog/MainView.xaml

@@ -139,6 +139,9 @@
       <TabItem Header="Pointers">
         <pages:PointersPage />
       </TabItem>
+      <TabItem Header="Popups">
+        <pages:PopupsPage />
+      </TabItem>
       <TabItem Header="ProgressBar">
         <pages:ProgressBarPage />
       </TabItem>

samples/ControlCatalog/Pages/PopupsPage.xaml

@@ -0,0 +1,18 @@
+<UserControl x:Class="ControlCatalog.Pages.PopupsPage"
+             xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+
+  <StackPanel Orientation="Vertical" Spacing="10">
+
+    <TextBlock Text="'Light Dismiss' Popup" />
+    <Button Content="Show Popup" Click="ButtonLightDismiss_OnClick" />
+
+    <TextBlock Text="Popup that stays open" />
+    <Button Content="Show Popup" Click="ButtonPopupStaysOpen_OnClick" />
+
+    <TextBlock Text="TopMost popup that stays open" />
+    <Button Content="Show Popup" Click="ButtonTopMostPopupStaysOpen" />
+
+  </StackPanel>
+
+</UserControl>

samples/ControlCatalog/Pages/PopupsPage.xaml.cs

@@ -0,0 +1,88 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Interactivity;
+using Avalonia.Markup.Xaml;
+
+namespace ControlCatalog.Pages;
+
+public class PopupsPage : UserControl
+{
+    private Popup? _popup;
+    private Popup? _topMostPopup;
+    public PopupsPage()
+    {
+        InitializeComponent();
+
+    }
+
+    private void InitializeComponent()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    private void ButtonLightDismiss_OnClick(object sender, RoutedEventArgs e)
+    {
+        new Popup
+        {
+            Placement = PlacementMode.Bottom,
+            PlacementTarget = sender as Button,
+            Child = new Panel { Margin = new Thickness(10) ,Children = { new TextBlock { Text = "Popup Content" } }},
+            IsLightDismissEnabled = true,
+            IsOpen = true
+        };
+    }
+
+    private void ButtonPopupStaysOpen_OnClick(object sender, RoutedEventArgs e)
+    {
+        if (_popup is not null)
+            return;
+
+        var closeButton = new Button { Content = "Click to Close" };
+        closeButton.Click += (o, args) =>
+        {
+            if (_popup is null)
+                return;
+            _popup.IsOpen = false;
+            _popup = null;
+        };
+        _popup = new Popup
+        {
+            Placement = PlacementMode.Bottom,
+            PlacementTarget = sender as Button,
+            Child = new Panel
+            {
+                Margin = new Thickness(10), Children = { closeButton }
+            },
+            IsLightDismissEnabled = false,
+            IsOpen = true
+        };
+    }
+
+    private void ButtonTopMostPopupStaysOpen(object sender, RoutedEventArgs e)
+    {
+        if (_topMostPopup is not null)
+            return;
+
+        var closeButton = new Button { Content = "Click to Close" };
+        closeButton.Click += (o, args) =>
+        {
+            if (_topMostPopup is null)
+                return;
+            _topMostPopup.IsOpen = false;
+            _topMostPopup = null;
+        };
+        _topMostPopup = new Popup
+        {
+            Placement = PlacementMode.Bottom,
+            PlacementTarget = sender as Button,
+            Child = new Panel
+            {
+                Margin = new Thickness(10), Children = { closeButton }
+            },
+            IsLightDismissEnabled = false,
+            Topmost = true,
+            IsOpen = true
+        };
+    }
+}

src/Avalonia.Controls/Primitives/PopupRoot.cs

@@ -154,7 +154,11 @@ void IPopupHost.ConfigurePosition(PopupPositionRequest request)
             UpdatePosition();
         }
 
-        public void SetChild(Control? control) => Content = control;
+        public void SetChild(Control? control)
+        {
+            PlatformImpl?.SetTopmost(Topmost);
+            Content = control;
+        }
 
         public void TakeFocus() => PlatformImpl?.TakeFocus();
 

src/Windows/Avalonia.Win32/PopupImpl.cs

@@ -53,8 +53,7 @@ protected override IntPtr CreateWindowOverride(ushort atom)
                 UnmanagedMethods.WindowStyles.WS_CLIPCHILDREN;
 
             UnmanagedMethods.WindowStyles exStyle =
-                UnmanagedMethods.WindowStyles.WS_EX_TOOLWINDOW |
-                UnmanagedMethods.WindowStyles.WS_EX_TOPMOST;
+                UnmanagedMethods.WindowStyles.WS_EX_TOOLWINDOW;
 
             var result = UnmanagedMethods.CreateWindowEx(
                 (int)exStyle,