NativeWebDialog.DefaultBackground integration

src/Avalonia.Controls.WebView.Core/Android/AndroidNativeWebViewDialog.cs

@@ -21,7 +21,7 @@ internal class AndroidNativeWebViewDialog(Action<WebViewEnvironmentRequestedEven
 
     public IWebViewAdapter? TryGetAdapter() => AndroidWebViewDialogActivity.WebViewRegistry.Get(_webViewId);
 
-    public Color DefaultBackground { get; set; }
+    public Color DefaultBackground { get; set; } = Colors.White;
     public string? Title { get; set; }
 
     public bool CanUserResize { get => false; set { } }

src/Avalonia.Controls.WebView.Core/Browser/BrowserWindowNativeWebViewDialog.cs

@@ -21,7 +21,7 @@ internal class BrowserWindowNativeWebViewDialog(Action<WebViewEnvironmentRequest
     private IWebViewAdapter? _adapter;
     private Action? _unsubClose;
     private string? _title;
-    private Color _defaultBackground;
+    private Color _defaultBackground = Colors.White;
     private bool _disposed;
 
     public IWebViewAdapter? TryGetAdapter() => _adapter;

src/Avalonia.Controls.WebView.Core/Gtk/GtkInterop.cs

@@ -142,7 +142,7 @@ internal static partial IntPtr webkit_website_data_manager_new(
     public static extern nint webkit_web_view_run_javascript_finish(IntPtr webView, IntPtr result, GError** error);
 
     [DllImport(LibWebKit)]
-    public static extern void webkit_web_view_set_background_color(IntPtr webView, GdkRGBA color);
+    public static extern void webkit_web_view_set_background_color(IntPtr webView, GdkRGBA* color);
 
     [DllImport(LibWebKit)]
     public static extern void webkit_javascript_result_unref(IntPtr jsResult);

src/Avalonia.Controls.WebView.Core/Gtk/GtkNativeWebViewDialog.cs

@@ -25,7 +25,33 @@ internal sealed class GtkNativeWebViewDialog : INativeWebViewDialog, IGtkWebView
     private bool _isShown;
 
     private sealed class DialogGtkWebViewAdapter(GtkWebViewEnvironmentRequestedEventArgs args)
-        : GtkWebViewAdapter(args);
+        : GtkWebViewAdapter(args)
+    {
+        public override Color DefaultBackground
+        {
+            set
+            {
+                var rgba = new GdkRGBA
+                {
+                    red = value.R / 255.0,
+                    green = value.G / 255.0,
+                    blue = value.B / 255.0,
+                    alpha = value.A / 255.0
+                };
+                RunOnGlibThreadAsync(() =>
+                {
+                    if (WebViewHandle == IntPtr.Zero)
+                        return;
+
+                    unsafe
+                    {
+                        var color = rgba;
+                        webkit_web_view_set_background_color(WebViewHandle, &color);
+                    }
+                });
+            }
+        }
+    }
 
     private GtkNativeWebViewDialog(GtkWebViewEnvironmentRequestedEventArgs args)
     {

src/Avalonia.Controls.WebView.Core/Headless/HeadlessWebViewAdapter.cs

@@ -66,11 +66,14 @@ public Uri Source
 
     public Color DefaultBackground
     {
-        set
-        {
-        }
+        set => LastDefaultBackground = value;
     }
 
+    /// <summary>
+    /// Last value assigned to <see cref="DefaultBackground"/>, useful with unit testing.
+    /// </summary>
+    internal Color? LastDefaultBackground { get; private set; }
+
     public string? UserAgent { get => null; set { } }
 
     public void SizeChanged(PixelSize containerSize)

src/Avalonia.Controls.WebView/NativeWebDialog.cs

@@ -7,6 +7,8 @@
 using AvPlatform = Avalonia.Platform;
 using Core = Avalonia.Controls;
 using IPlatformHandle = Avalonia.Platform.IPlatformHandle;
+using Color = Avalonia.Media.Color;
+using Colors = Avalonia.Media.Colors;
 #if WPF
 using AvaloniaUI.Xpf.WpfAbstractions;
 using Window = System.Windows.Window;
@@ -38,6 +40,7 @@ public class NativeWebDialog : Core.IWebView, Core.IWebViewHolder, IDisposable
         private bool? _initialCanUserResize;
         private PixelSize? _initialSize;
         private PixelPoint? _initialPosition;
+        private Color? _initialDefaultBackground;
         private bool _disposed;
         private bool _dialogInitialized;
 
@@ -276,6 +279,24 @@ public bool CanUserResize
             }
         }
 
+        /// <summary>
+        /// Gets or sets the background color of the dialog and of the webview hosted inside of it.
+        /// If null, the owner background is used, falling back to white.
+        /// </summary>
+        public Color? DefaultBackground
+        {
+            get => _initialDefaultBackground;
+            set
+            {
+                _initialDefaultBackground = value;
+                if (value is { } background
+                    && TryGetImpl() is { } impl)
+                {
+                    impl.DefaultBackground = background;
+                }
+            }
+        }
+
         /// <inheritdoc cref="Core.INativeWebViewDialog.Closing"/>
         public event EventHandler? Closing;
         /// <inheritdoc cref="Core.INativeWebViewDialog.Show()"/>
@@ -295,6 +316,12 @@ public async void Show(TopLevel owner)
         {
             var impl = await GetOrInitialize();
 
+            // Not stored in _initialDefaultBackground, so the owner is still resolved again on the next Show call.
+            if (_initialDefaultBackground is null)
+            {
+                impl.DefaultBackground = GetOwnerBackground(owner);
+            }
+
 #if WPF
             var avTopLevel = XpfWpfAbstraction.GetAvaloniaTopLevelForWindow(owner);
 #elif AVALONIA
@@ -317,6 +344,18 @@ public async void Show(TopLevel owner)
             }
         }
 
+#if WPF
+        private static Color GetOwnerBackground(Window owner) =>
+            owner.Background is System.Windows.Media.SolidColorBrush solid ?
+                new Color(solid.Color.A, solid.Color.R, solid.Color.G, solid.Color.B) :
+                Colors.White;
+#elif AVALONIA
+        private static Color GetOwnerBackground(TopLevel owner) =>
+            owner.Background is Media.ISolidColorBrush solid ?
+                solid.Color :
+                Colors.White;
+#endif
+
         private async Task<Core.INativeWebViewDialog> GetOrInitialize()
         {
             if (TryGetImpl() is { } impl)
@@ -439,6 +478,8 @@ private async Task Initialize()
                 dialogImpl.Move(position.X, position.Y);
             if (_initialSize is { } size)
                 dialogImpl.Resize(size.Width, size.Height);
+            if (_initialDefaultBackground is { } background)
+                dialogImpl.DefaultBackground = background;
 
             _implTcs.SetResult(dialogImpl);
 

src/Avalonia.Controls.WebView/WebAuthenticationBroker.cs

@@ -146,7 +146,7 @@ private static NativeWebDialog DefaultFactory()
         {
             var dialog = new NativeWebDialog();
             dialog.Title = "Authentication";
-            dialog.CanUserResize = false;
+            dialog.CanUserResize = true;
             dialog.Resize(600, 700);
             return dialog;
         }

src/Avalonia.Controls.WebView/WindowNativeWebViewDialog.cs

@@ -56,7 +56,10 @@ async void CompleteAdapter()
 
                 _controlHostImpl.AdapterCreated += (_, adapter) =>
                 {
-                    adapter.DefaultBackground = _initialDefaultBackground ?? Colors.Transparent;
+                    // Opaque by default, so web pages without their own background stay readable with a dark theme.
+                    var background = _initialDefaultBackground ?? Colors.White;
+                    adapter.DefaultBackground = background;
+                    ApplyWindowBackground(background);
                     AdapterCreated?.Invoke(this, new Core.WebViewAdapterEventArgs(adapter));
                 };
                 _controlHostImpl.AdapterDestroyed += (_, adapter) => AdapterDestroyed?.Invoke(this, new Core.WebViewAdapterEventArgs(adapter));
@@ -77,17 +80,26 @@ public Color DefaultBackground
         {
             set
             {
+                _initialDefaultBackground = value;
                 if (_controlHostImpl?.TryGetAdapter() is { } adapter)
                 {
                     adapter.DefaultBackground = value;
                 }
-                else
-                {
-                    _initialDefaultBackground = value;
-                }
+                ApplyWindowBackground(value);
             }
         }
 
+        // The webview may be composited into the window (or transparent on macOS),
+        // so the window itself has to use the same color.
+        private void ApplyWindowBackground(Color color)
+        {
+#if WPF
+            Background = new SolidColorBrush(System.Windows.Media.Color.FromArgb(color.A, color.R, color.G, color.B));
+#elif AVALONIA
+            Background = new Media.SolidColorBrush(color);
+#endif
+        }
+
         public void Dispose() => Close();
 
         event EventHandler? Core.INativeWebViewDialog.Closing

tests/Avalonia.Controls.WebView.Tests/NativeWebDialogTests.cs

@@ -1,4 +1,6 @@
-using Avalonia.Headless.XUnit;
+using Avalonia.Controls.Headless;
+using Avalonia.Headless.XUnit;
+using Avalonia.Media;
 using System;
 using System.Threading.Tasks;
 using Xunit;
@@ -56,6 +58,72 @@ public void Should_Set_And_Reflect_Position_And_Size()
         Assert.Equal(60, underlying.Position.Y);
     }
 
+    [AvaloniaFact]
+    public async Task Should_Use_White_Background_Without_Owner()
+    {
+        var dialog = new NativeWebDialog();
+        dialog.Show();
+
+        await WaitForAdapterCreation(dialog);
+
+        Assert.Null(dialog.DefaultBackground);
+        Assert.Equal(Colors.White, GetAdapterBackground(dialog));
+        Assert.Equal(Colors.White, GetWindowBackground(dialog));
+    }
+
+    [AvaloniaFact]
+    public async Task Should_Use_Owner_Background_When_Not_Set()
+    {
+        var owner = new Window { Background = new SolidColorBrush(Colors.Blue) };
+        owner.Show();
+
+        var dialog = new NativeWebDialog();
+        dialog.Show(owner);
+
+        await WaitForAdapterCreation(dialog);
+
+        Assert.Null(dialog.DefaultBackground);
+        Assert.Equal(Colors.Blue, GetAdapterBackground(dialog));
+        Assert.Equal(Colors.Blue, GetWindowBackground(dialog));
+    }
+
+    [AvaloniaFact]
+    public async Task Should_Set_DefaultBackground_Before_Show()
+    {
+        var owner = new Window { Background = new SolidColorBrush(Colors.Blue) };
+        owner.Show();
+
+        var dialog = new NativeWebDialog();
+        dialog.DefaultBackground = Colors.Red;
+        dialog.Show(owner);
+
+        await WaitForAdapterCreation(dialog);
+
+        // Explicit value wins over the owner background.
+        Assert.Equal(Colors.Red, dialog.DefaultBackground);
+        Assert.Equal(Colors.Red, GetAdapterBackground(dialog));
+        Assert.Equal(Colors.Red, GetWindowBackground(dialog));
+    }
+
+    [AvaloniaFact]
+    public async Task Should_Set_DefaultBackground_After_Show()
+    {
+        var dialog = new NativeWebDialog();
+        dialog.Show();
+
+        await WaitForAdapterCreation(dialog);
+
+        dialog.DefaultBackground = Colors.Red;
+        Assert.Equal(Colors.Red, GetAdapterBackground(dialog));
+        Assert.Equal(Colors.Red, GetWindowBackground(dialog));
+    }
+
+    private static Color? GetAdapterBackground(NativeWebDialog dialog) =>
+        ((HeadlessWebViewAdapter)dialog.TryGetWebViewPlatformHandle()!).LastDefaultBackground;
+
+    private static Color? GetWindowBackground(NativeWebDialog dialog) =>
+        (dialog.TryGetWindow()!.Background as ISolidColorBrush)?.Color;
+
     [AvaloniaFact]
     public void Should_Expose_Platform_Handle()
     {