Fix startup crash in HC theme: use Fluent-compatible ThemeMode before MainWindow creation

Agent-Logs-Url: https://github.com/microsoft/PowerToys/sessions/5e9c2db1-f7e8-406e-9f8b-9c413fab9660

Co-authored-by: MuyuanMS <116717757+MuyuanMS@users.noreply.github.com>

src/modules/launcher/PowerLauncher/App.xaml

@@ -7,7 +7,6 @@
     <Application.Resources>
         <ResourceDictionary>
             <ResourceDictionary.MergedDictionaries>
-                <ResourceDictionary Source="pack://application:,,,/PresentationFramework.Fluent;component/Themes/Fluent.xaml" />
                 <ResourceDictionary Source="pack://application:,,,/Styles/Styles.xaml" />
             </ResourceDictionary.MergedDictionaries>
         </ResourceDictionary>

src/modules/launcher/PowerLauncher/App.xaml.cs

@@ -153,6 +153,23 @@ namespace PowerLauncher
                 _stringMatcher.UserSettingSearchPrecision = _settings.QuerySearchPrecision;
 
                 _mainVM = new MainViewModel(_settings, NativeThreadCTS.Token);
+
+                // Set ThemeMode before MainWindow creation so Fluent static resources
+                // (e.g. DefaultTextBoxStyle, CaptionTextBlockStyle referenced via BasedOn)
+                // are available for XAML parsing. ThemeMode.None cannot be used here even for
+                // high-contrast themes, because WPF's BasedOn StaticResource lookups require
+                // the named Fluent resources to exist at parse time.
+                // HighContrastWhite → Light (visually closer); other HC → Dark fallback.
+                // ThemeManager.SetSystemTheme will apply the correct ThemeMode (None for HC)
+                // once the window is ready.
+                var themeHelper = new ThemeHelper();
+                var initialTheme = themeHelper.DetermineTheme(_settings.Theme);
+#pragma warning disable WPF0001
+                Application.Current.ThemeMode = initialTheme is Theme.Light or Theme.HighContrastWhite
+                    ? ThemeMode.Light
+                    : ThemeMode.Dark;
+#pragma warning restore WPF0001
+
                 _mainWindow = new MainWindow(_settings, _mainVM, NativeThreadCTS.Token);
                 _themeManager = new ThemeManager(_settings, _mainWindow);
                 API = new PublicAPIInstance(_settingsVM, _mainVM, _alphabet, _themeManager);

src/modules/launcher/PowerLauncher/Helper/ThemeManager.cs

@@ -23,6 +23,7 @@ namespace PowerLauncher.Helper
         private readonly ThemeHelper _themeHelper = new();
 
         private bool _disposed;
+        private bool _isHighContrastMode;
         private CancellationTokenSource _themeUpdateTokenSource;
         private const int MaxRetries = 5;
         private const int InitialDelayMs = 2000;
@@ -31,6 +32,15 @@ namespace PowerLauncher.Helper
 
         public event Common.UI.ThemeChangedHandler ThemeChanged;
 
+        internal static ThemeMode GetThemeMode(Theme theme) => theme switch
+        {
+            Theme.Light => ThemeMode.Light,
+            Theme.Dark => ThemeMode.Dark,
+            Theme.HighContrastBlack or Theme.HighContrastWhite or Theme.HighContrastOne or
+                Theme.HighContrastTwo => ThemeMode.None,
+            _ => ThemeMode.Dark,
+        };
+
         public ThemeManager(PowerToysRunSettings settings, MainWindow mainWindow)
         {
             _settings = settings;
@@ -51,14 +61,22 @@ namespace PowerLauncher.Helper
         {
             _mainWindow.Background = !OSVersionHelper.IsWindows11() ? SystemColors.WindowBrush : null;
 
-            // Need to disable WPF0001 since setting Application.Current.ThemeMode is experimental
-            // https://learn.microsoft.com/en-us/dotnet/desktop/wpf/whats-new/net90#set-in-code
-#pragma warning disable WPF0001
-            Application.Current.ThemeMode = theme == Theme.Light ? ThemeMode.Light : ThemeMode.Dark;
-#pragma warning restore WPF0001
-
             if (theme is Theme.Dark or Theme.Light)
             {
+                // When returning from a high-contrast theme, clear the high-contrast resource
+                // dictionaries that were applied to the window.
+                if (_isHighContrastMode)
+                {
+                    _mainWindow.Resources.MergedDictionaries.Clear();
+                    _isHighContrastMode = false;
+                }
+
+                // Need to disable WPF0001 since setting Application.Current.ThemeMode is experimental
+                // https://learn.microsoft.com/en-us/dotnet/desktop/wpf/whats-new/net90#set-in-code
+#pragma warning disable WPF0001
+                Application.Current.ThemeMode = GetThemeMode(theme);
+#pragma warning restore WPF0001
+
                 if (!OSVersionHelper.IsWindows11())
                 {
                     // Apply background only on Windows 10
@@ -71,6 +89,14 @@ namespace PowerLauncher.Helper
             }
             else
             {
+                // For high-contrast themes, disable WPF's Fluent theme manager to avoid conflicts
+                // with the custom high-contrast resource dictionaries.
+#pragma warning disable WPF0001
+                Application.Current.ThemeMode = GetThemeMode(theme);
+#pragma warning restore WPF0001
+
+                _isHighContrastMode = true;
+
                 string styleThemeString = theme switch
                 {
                     Theme.HighContrastOne => "Themes/HighContrast1.xaml",

src/modules/launcher/PowerLauncher/Styles/Styles.xaml

@@ -50,7 +50,7 @@
     </Style>
 
     <Style x:Key="SubtleButtonStyle" TargetType="{x:Type Button}">
-        <Setter Property="FocusVisualStyle" Value="{StaticResource FocusVisual}" />
+        <Setter Property="FocusVisualStyle" Value="{DynamicResource FocusVisual}" />
         <Setter Property="Foreground" Value="{DynamicResource TextFillColorPrimaryBrush}" />
         <Setter Property="BorderThickness" Value="0" />
         <Setter Property="Focusable" Value="False" />

src/modules/launcher/Wox.Test/ThemeManagerTest.cs

@@ -0,0 +1,26 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Windows;
+using ManagedCommon;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using PowerLauncher.Helper;
+
+namespace Wox.Test;
+
+[TestClass]
+public class ThemeManagerTest
+{
+    [DataTestMethod]
+    [DataRow(Theme.Light, ThemeMode.Light)]
+    [DataRow(Theme.Dark, ThemeMode.Dark)]
+    [DataRow(Theme.HighContrastBlack, ThemeMode.None)]
+    [DataRow(Theme.HighContrastWhite, ThemeMode.None)]
+    [DataRow(Theme.HighContrastOne, ThemeMode.None)]
+    [DataRow(Theme.HighContrastTwo, ThemeMode.None)]
+    public void GetThemeMode_ReturnsExpectedThemeMode(Theme theme, ThemeMode expectedThemeMode)
+    {
+        Assert.AreEqual(expectedThemeMode, ThemeManager.GetThemeMode(theme));
+    }
+}