Fix dialog losing focus when changing WindowState

src/MaterialDesignThemes.Wpf/DialogHost.cs

@@ -64,6 +64,8 @@ public class DialogHost : ContentControl
     private DialogClosingEventHandler? _attachedDialogClosingEventHandler;
     private DialogClosedEventHandler? _attachedDialogClosedEventHandler;
     private IInputElement? _restoreFocusDialogClose;
+    private IInputElement? _lastFocusedDialogElement;
+    private WindowState _previousWindowState;
     private Action? _currentSnackbarMessageQueueUnPauseAction;
 
     static DialogHost()
@@ -370,6 +372,7 @@ private static void IsOpenPropertyChangedCallback(DependencyObject dependencyObj
 
         dialogHost.CurrentSession = new DialogSession(dialogHost);
         var window = Window.GetWindow(dialogHost);
+        dialogHost.ListenForWindowStateChanged(window);
         if (!dialogHost.IsRestoreFocusDisabled)
         {
             dialogHost._restoreFocusDialogClose = window != null ? FocusManager.GetFocusedElement(window) : null;
@@ -395,7 +398,8 @@ private static void IsOpenPropertyChangedCallback(DependencyObject dependencyObj
 
         //https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/issues/187
         //totally not happy about this, but on immediate validation we can get some weird looking stuff...give WPF a kick to refresh...
-        Task.Delay(300).ContinueWith(t => dialogHost.Dispatcher.BeginInvoke(DispatcherPriority.Background, new Action(() => {
+        Task.Delay(300).ContinueWith(t => dialogHost.Dispatcher.BeginInvoke(DispatcherPriority.Background, new Action(() =>
+        {
             CommandManager.InvalidateRequerySuggested();
             //Delay focusing the popup until after the animation has some time, Issue #2912
             UIElement? child = dialogHost.FocusPopup();
@@ -405,6 +409,50 @@ private static void IsOpenPropertyChangedCallback(DependencyObject dependencyObj
         })));
     }
 
+
+    private void ListenForWindowStateChanged(Window? window)
+    {
+        window ??= Window.GetWindow(this);
+
+        if (window is not null)
+        {
+            window.StateChanged += Window_StateChanged;
+        }
+    }
+
+    private void Window_StateChanged(object? sender, EventArgs e)
+    {
+        if (sender is not Window window)
+        {
+            return;
+        }
+
+        var windowState = window.WindowState;
+        if (windowState == WindowState.Minimized)
+        {
+            _lastFocusedDialogElement = FocusManager.GetFocusedElement(window);
+            _previousWindowState = windowState;
+            return;
+        }
+
+        // We only need to focus anything manually if the window changes state from Minimized --> (Normal or Maximized)
+        // Going from Normal --> Maximized (and vice versa) is fine since the focus is already kept correctly
+        if (IsWindowRestoredFromMinimized() && IsLastFocusedDialogElementFocusable())
+        {
+            // Kinda hacky, but without a delay the focus doesn't always get set correctly because the Focus() method fires too early
+            Task.Delay(50).ContinueWith(_ => this.Dispatcher.BeginInvoke(DispatcherPriority.Background, new Action(() =>
+            {
+                _lastFocusedDialogElement!.Focus();
+            })));
+        }
+        _previousWindowState = windowState;
+
+        bool IsWindowRestoredFromMinimized() => (windowState == WindowState.Normal || windowState == WindowState.Maximized) &&
+                                                _previousWindowState == WindowState.Minimized;
+
+        bool IsLastFocusedDialogElementFocusable() => _lastFocusedDialogElement is UIElement { Focusable: true, IsVisible: true };
+    }
+
     /// <summary>
     /// Returns a DialogSession for the currently open dialog for managing it programmatically. If no dialog is open, CurrentSession will return null
     /// </summary>

tests/MaterialDesignThemes.UITests/Samples/DialogHost/WithMultipleTextBoxes.xaml

@@ -0,0 +1,28 @@
+<UserControl x:Class="MaterialDesignThemes.UITests.Samples.DialogHost.WithMultipleTextBoxes"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:local="clr-namespace:MaterialDesignThemes.UITests.Samples.DialogHost"
+             xmlns:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             d:DesignHeight="450"
+             d:DesignWidth="800"
+             mc:Ignorable="d">
+  <materialDesign:DialogHost x:Name="SampleDialogHost" Loaded="DialogHost_Loaded">
+    <materialDesign:DialogHost.DialogContent>
+      <StackPanel Margin="16"
+                  HorizontalAlignment="Center"
+                  VerticalAlignment="Center">
+        <TextBox x:Name="TextBoxOne"
+                 MinWidth="200"
+                 Text="One" />
+        <TextBox x:Name="TextBoxTwo"
+                 MinWidth="200"
+                 Text="Two" />
+        <TextBox x:Name="TextBoxThree"
+                 MinWidth="200"
+                 Text="Three" />
+      </StackPanel>
+    </materialDesign:DialogHost.DialogContent>
+  </materialDesign:DialogHost>
+</UserControl>

tests/MaterialDesignThemes.UITests/Samples/DialogHost/WithMultipleTextBoxes.xaml.cs

@@ -0,0 +1,16 @@
+namespace MaterialDesignThemes.UITests.Samples.DialogHost;
+
+/// <summary>
+/// Interaction logic for WithMultipleTextBoxes.xaml
+/// </summary>
+public partial class WithMultipleTextBoxes : UserControl
+{
+    public WithMultipleTextBoxes()
+    {
+        InitializeComponent();
+    }
+    private void DialogHost_Loaded(object sender, RoutedEventArgs e)
+    {
+        SampleDialogHost.IsOpen = true;
+    }
+}

tests/MaterialDesignThemes.UITests/WPF/DialogHosts/DialogHostTests.cs

@@ -314,7 +314,6 @@ public async Task CornerRadius_AppliedToContentCoverBorder_WhenSetOnEmbeddedDial
         await Wait.For(async () =>
         {
             var contentCoverBorder = await dialogHost.GetElement<Border>("ContentCoverBorder");
-
             await Assert.That((await contentCoverBorder.GetCornerRadius()).TopLeft).IsEqualTo(1);
             await Assert.That((await contentCoverBorder.GetCornerRadius()).TopRight).IsEqualTo(2);
             await Assert.That((await contentCoverBorder.GetCornerRadius()).BottomRight).IsEqualTo(3);
@@ -500,7 +499,6 @@ public async Task DialogHost_WithComboBox_CanSelectItem()
         var comboBox = await dialogHost.GetElement<ComboBox>("TargetedPlatformComboBox");
         await Task.Delay(500, TestContext.Current!.CancellationToken);
         await comboBox.LeftClick();
-
         var item = await Wait.For(() => comboBox.GetElement<ComboBoxItem>("TargetItem"));
         await Task.Delay(TimeSpan.FromSeconds(1));
         await item.LeftClick();
@@ -514,4 +512,40 @@ await Wait.For(async () =>
 
         recorder.Success();
     }
+
+    [Test]
+    [Description("Issue 3434")]
+    [Arguments(WindowState.Minimized, WindowState.Maximized)]
+    [Arguments(WindowState.Minimized, WindowState.Normal)]
+    [Arguments(WindowState.Maximized, WindowState.Normal)]
+	public async Task DialogHost_WhenWindowStateChanges_FocusedElementStaysFocused(WindowState firstWindowState, WindowState secondWindowState)
+	{
+		await using var recorder = new TestRecorder(App);
+
+		var dialogHost = (await LoadUserControl<WithMultipleTextBoxes>()).As<DialogHost>();
+		await Task.Delay(400, TestContext.Current!.CancellationToken);
+
+        // Select the second TextBox
+        var tbTwo = await dialogHost.GetElement<TextBox>("TextBoxTwo");
+        await tbTwo.MoveKeyboardFocus();
+		await Assert.That(await tbTwo.GetIsFocused()).IsTrue();
+
+		// First state
+		await dialogHost.RemoteExecute(SetStateOfParentWindow, firstWindowState);
+		await Task.Delay(400, TestContext.Current!.CancellationToken);
+        // Second state
+        await dialogHost.RemoteExecute(SetStateOfParentWindow, secondWindowState);
+		await Task.Delay(400, TestContext.Current!.CancellationToken);
+
+		// After changing state of the window the previously focused element should be focused again
+		await Assert.That(await tbTwo.GetIsFocused()).IsTrue();
+		recorder.Success();
+
+		static object SetStateOfParentWindow(DialogHost dialogHost, WindowState state)
+        {
+            var window = Window.GetWindow(dialogHost);
+            window.WindowState = state;
+            return null!;
+        }
+	}
 }