Fluent2: use OS accent shades on Windows, drop WinUI's forced MinWidth

PlatformColorValues gains six optional accent shade slots
(AccentColorDark1-3/Light1-3); the Win32 backend fills them from the
UISettings object it already holds, and SystemAccentColors prefers them
over HSL-computed shades, so the palette matches Windows exactly.

CheckBox/RadioButton no longer force WinUI's 120px MinWidth (v1 never
had it); 8px trailing padding compensates, and the CheckBoxMinWidth/
RadioButtonMinWidth resources stay as override knobs. Both changes
follow FluentAvalonia's (MIT) lead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Avalonia.Base/Platform/PlatformColorValues.cs

@@ -61,6 +61,37 @@ public record PlatformColorValues
         init => _accentColor3 = value;
     }
 
+    /// <summary>
+    /// Dark shade 1 of <see cref="AccentColor1"/>, when the platform provides one (Windows does).
+    /// Null when the platform only reports the base accent color; consumers are expected to compute a shade instead.
+    /// </summary>
+    public Color? AccentColorDark1 { get; init; }
+
+    /// <summary>
+    /// Dark shade 2 of <see cref="AccentColor1"/>, when the platform provides one. See <see cref="AccentColorDark1"/>.
+    /// </summary>
+    public Color? AccentColorDark2 { get; init; }
+
+    /// <summary>
+    /// Dark shade 3 of <see cref="AccentColor1"/>, when the platform provides one. See <see cref="AccentColorDark1"/>.
+    /// </summary>
+    public Color? AccentColorDark3 { get; init; }
+
+    /// <summary>
+    /// Light shade 1 of <see cref="AccentColor1"/>, when the platform provides one. See <see cref="AccentColorDark1"/>.
+    /// </summary>
+    public Color? AccentColorLight1 { get; init; }
+
+    /// <summary>
+    /// Light shade 2 of <see cref="AccentColor1"/>, when the platform provides one. See <see cref="AccentColorDark1"/>.
+    /// </summary>
+    public Color? AccentColorLight2 { get; init; }
+
+    /// <summary>
+    /// Light shade 3 of <see cref="AccentColor1"/>, when the platform provides one. See <see cref="AccentColorDark1"/>.
+    /// </summary>
+    public Color? AccentColorLight3 { get; init; }
+
     public PlatformColorValues()
     {
         AccentColor1 = DefaultAccent;

src/Avalonia.Themes.Fluent2/Accents/SystemAccentColors.cs

@@ -20,7 +20,8 @@ internal sealed class SystemAccentColors : ResourceProvider
     private static readonly Color s_defaultSystemAccentColor = Color.FromRgb(0, 120, 215);
     // Static shade values from WinUI (matching the Uno/WinUI SystemResources defaults). Used when
     // the platform provides no accent color, so the out-of-the-box palette matches WinUI exactly.
-    // OS-provided accent colors get computed shades instead.
+    // When the platform reports the OS-computed shades (Windows does, via UISettings) those are
+    // used verbatim; otherwise an OS-provided base accent gets HSL-computed shades.
     private static readonly Color s_defaultSystemAccentColorDark1 = Color.FromUInt32(0xFF005A9E);
     private static readonly Color s_defaultSystemAccentColorDark2 = Color.FromUInt32(0xFF004275);
     private static readonly Color s_defaultSystemAccentColorDark3 = Color.FromUInt32(0xFF002642);
@@ -117,11 +118,22 @@ internal sealed class SystemAccentColors : ResourceProvider
         {
             _invalidateColors = false;
 
-            var platformSettings = GetFromOwner(Owner);
+            var colorValues = GetFromOwner(Owner)?.GetColorValues();
 
-            _systemAccentColor = platformSettings?.GetColorValues().AccentColor1 ?? s_defaultSystemAccentColor;
+            _systemAccentColor = colorValues?.AccentColor1 ?? s_defaultSystemAccentColor;
 
-            if (_systemAccentColor == s_defaultSystemAccentColor)
+            if (colorValues is
+                {
+                    AccentColorDark1: { } dark1, AccentColorDark2: { } dark2, AccentColorDark3: { } dark3,
+                    AccentColorLight1: { } light1, AccentColorLight2: { } light2, AccentColorLight3: { } light3
+                })
+            {
+                // The platform reported the OS-computed shades (Windows, via UISettings);
+                // use them verbatim so the palette matches the OS exactly.
+                (_systemAccentColorDark1, _systemAccentColorDark2, _systemAccentColorDark3) = (dark1, dark2, dark3);
+                (_systemAccentColorLight1, _systemAccentColorLight2, _systemAccentColorLight3) = (light1, light2, light3);
+            }
+            else if (_systemAccentColor == s_defaultSystemAccentColor)
             {
                 // Platforms without a real accent color report the default; use WinUI's static
                 // shade values so the out-of-the-box palette matches WinUI exactly.

src/Avalonia.Themes.Fluent2/Controls/CheckBox.xaml

@@ -14,11 +14,15 @@
 
   <StreamGeometry x:Key="CheckMarkPathData">M5.5 10.586 1.707 6.793A1 1 0 0 0 .293 8.207l4.5 4.5a1 1 0 0 0 1.414 0l11-11A1 1 0 0 0 15.793.293L5.5 10.586Z</StreamGeometry>
   <x:Double x:Key="CheckBoxMinHeight">32</x:Double>
-  <x:Double x:Key="CheckBoxMinWidth">120</x:Double>
+  <!-- WinUI forces MinWidth=120, which breaks desktop form layouts (and v1 never had it).
+       Deliberately 0 here; override this resource to restore the WinUI metric. -->
+  <x:Double x:Key="CheckBoxMinWidth">0</x:Double>
 
   <!-- WinUI reference: CheckBox_themeresources.xaml (DefaultCheckBoxStyle), tag winui3/release/1.5.2 -->
   <ControlTheme x:Key="{x:Type CheckBox}" TargetType="CheckBox">
-    <Setter Property="Padding" Value="8,5,0,0" />
+    <!-- WinUI pads 8,5,0,0 and relies on MinWidth for trailing space; with MinWidth dropped,
+         8px right padding keeps a sane hit target (same amendment FluentAvalonia made). -->
+    <Setter Property="Padding" Value="8,5,8,0" />
     <Setter Property="HorizontalAlignment" Value="Left" />
     <Setter Property="VerticalAlignment" Value="Center" />
     <Setter Property="HorizontalContentAlignment" Value="Left" />

src/Avalonia.Themes.Fluent2/Controls/RadioButton.xaml

@@ -12,7 +12,9 @@
   </Design.PreviewWith>
 
   <x:Double x:Key="RadioButtonMinHeight">32</x:Double>
-  <x:Double x:Key="RadioButtonMinWidth">120</x:Double>
+  <!-- WinUI forces MinWidth=120, which breaks desktop form layouts (and v1 never had it).
+       Deliberately 0 here; override this resource to restore the WinUI metric. -->
+  <x:Double x:Key="RadioButtonMinWidth">0</x:Double>
   <x:Double x:Key="RadioButtonCheckGlyphSize">12</x:Double>
   <x:Double x:Key="RadioButtonCheckGlyphPointerOverSize">14</x:Double>
   <x:Double x:Key="RadioButtonCheckGlyphPressedOverSize">10</x:Double>
@@ -28,7 +30,9 @@
     <Setter Property="Foreground" Value="{DynamicResource RadioButtonForeground}" />
     <Setter Property="BorderBrush" Value="{DynamicResource RadioButtonBorderBrush}" />
     <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
-    <Setter Property="Padding" Value="8,6,0,0" />
+    <!-- WinUI pads 8,6,0,0 and relies on MinWidth for trailing space; with MinWidth dropped,
+         8px right padding keeps a sane hit target (same amendment FluentAvalonia made). -->
+    <Setter Property="Padding" Value="8,6,8,0" />
     <Setter Property="MinWidth" Value="{DynamicResource RadioButtonMinWidth}" />
     <Setter Property="HorizontalAlignment" Value="Left" />
     <Setter Property="VerticalAlignment" Value="Center" />

src/Avalonia.Themes.Fluent2/README.md

@@ -48,6 +48,12 @@ Migrating from `Avalonia.Themes.Fluent`: change the package reference and swap
 | Scroll bars | 16 px rail | **12 px rail, 2 px collapsed thumb** |
 | Window background | AltHigh | **SolidBackgroundFillColorBase** (#F3F3F3/#202020) |
 | Default accent shades | HSL-computed | **WinUI static values** (when no OS accent) |
+| OS accent shades (Windows) | HSL-computed | **read from the OS** (`UISettings` Light1–3/Dark1–3) |
+
+One deliberate deviation from WinUI (following FluentAvalonia's lead): CheckBox
+and RadioButton do **not** get WinUI's forced 120 px `MinWidth` — they size to
+content plus 8 px trailing padding, as in v1. Set the `CheckBoxMinWidth` /
+`RadioButtonMinWidth` resources to restore the WinUI metric.
 
 The full WinUI 3 token family (`TextFillColor*`, `ControlFillColor*`,
 `SubtleFillColor*`, `ControlStrokeColor*`, `CardBackgroundFillColor*`,
@@ -64,8 +70,10 @@ legacy `SystemControl*` aliases.
 - No HighContrast variant yet (same as v1).
 - AnimatedIcon glyph animations are approximated with static glyphs and simple
   transitions.
-- OS-provided accent colors still use HSL-computed shades, which can deviate
-  slightly from Windows' palette algorithm.
+- On Windows the six accent shades (`SystemAccentColorDark1`…`Light3`) come
+  straight from the OS; on platforms that report only a base accent color
+  (macOS, Linux) the shades are HSL-computed and can deviate slightly from
+  Windows' palette algorithm.
 
 ## Compatibility tests
 

src/Windows/Avalonia.Win32/Win32PlatformSettings.cs

@@ -73,8 +73,14 @@ internal class Win32PlatformSettings : DefaultPlatformSettings
                     PlatformThemeVariant.Dark :
                     PlatformThemeVariant.Light,
                 ContrastPreference = ColorContrastPreference.NoPreference,
-                AccentColor1 = accent
-            };   
+                AccentColor1 = accent,
+                AccentColorDark1 = uiSettings.GetColorValue(UIColorType.AccentDark1).ToAvalonia(),
+                AccentColorDark2 = uiSettings.GetColorValue(UIColorType.AccentDark2).ToAvalonia(),
+                AccentColorDark3 = uiSettings.GetColorValue(UIColorType.AccentDark3).ToAvalonia(),
+                AccentColorLight1 = uiSettings.GetColorValue(UIColorType.AccentLight1).ToAvalonia(),
+                AccentColorLight2 = uiSettings.GetColorValue(UIColorType.AccentLight2).ToAvalonia(),
+                AccentColorLight3 = uiSettings.GetColorValue(UIColorType.AccentLight3).ToAvalonia()
+            };
         }
     }
     

tests/Avalonia.Themes.Fluent2.UnitTests/ControlDefaultsTests.cs

@@ -0,0 +1,50 @@
+using Avalonia.Controls;
+using Avalonia.Threading;
+
+namespace Avalonia.Themes.Fluent2.UnitTests;
+
+/// <summary>
+/// Guards the deliberate desktop-ergonomic deviations from the WinUI metrics.
+/// </summary>
+public class ControlDefaultsTests
+{
+    [AvaloniaFact]
+    public void CheckBox_and_RadioButton_do_not_force_winui_min_width()
+    {
+        var theme = new Fluent2Theme();
+        var app = Application.Current!;
+        app.Styles.Add(theme);
+        try
+        {
+            var checkBox = new CheckBox { Content = "Ok" };
+            var radioButton = new RadioButton { Content = "Ok" };
+            var window = new Window
+            {
+                Width = 400,
+                Height = 300,
+                Content = new StackPanel { Children = { checkBox, radioButton } },
+            };
+            window.Show();
+            try
+            {
+                Dispatcher.UIThread.RunJobs();
+
+                // WinUI forces MinWidth=120 on both; v1 never did, and neither does Fluent2.
+                Assert.Equal(0d, checkBox.MinWidth);
+                Assert.Equal(0d, radioButton.MinWidth);
+                Assert.True(checkBox.Bounds.Width < 120,
+                    $"CheckBox width {checkBox.Bounds.Width} should hug its content.");
+                Assert.True(radioButton.Bounds.Width < 120,
+                    $"RadioButton width {radioButton.Bounds.Width} should hug its content.");
+            }
+            finally
+            {
+                window.Close();
+            }
+        }
+        finally
+        {
+            app.Styles.Remove(theme);
+        }
+    }
+}

tests/Avalonia.Themes.Fluent2.UnitTests/PaletteTests.cs

@@ -1,5 +1,6 @@
 using Avalonia.Controls;
 using Avalonia.Media;
+using Avalonia.Platform;
 using Avalonia.Styling;
 
 namespace Avalonia.Themes.Fluent2.UnitTests;
@@ -65,6 +66,48 @@ public class PaletteTests
         }
     }
 
+    [AvaloniaFact]
+    public void Platform_provided_accent_shades_take_precedence_over_computed_ones()
+    {
+        var originalSettings = AvaloniaLocator.Current.GetService<IPlatformSettings>();
+        Assert.NotNull(originalSettings);
+        AvaloniaLocator.CurrentMutable.Bind<IPlatformSettings>().ToConstant(new ShadeReportingPlatformSettings());
+
+        var theme = new Fluent2Theme();
+        var app = Application.Current!;
+        app.Styles.Add(theme);
+        try
+        {
+            Assert.True(app.TryGetResource("SystemAccentColor", ThemeVariant.Light, out var accent));
+            Assert.Equal(Colors.Crimson, Assert.IsType<Color>(accent));
+
+            // The OS-reported shades are used verbatim, not HSL-computed from the base color.
+            Assert.True(app.TryGetResource("SystemAccentColorDark1", ThemeVariant.Light, out var dark1));
+            Assert.Equal(Colors.DarkRed, Assert.IsType<Color>(dark1));
+            Assert.True(app.TryGetResource("SystemAccentColorLight3", ThemeVariant.Light, out var light3));
+            Assert.Equal(Colors.MistyRose, Assert.IsType<Color>(light3));
+        }
+        finally
+        {
+            app.Styles.Remove(theme);
+            AvaloniaLocator.CurrentMutable.Bind<IPlatformSettings>().ToConstant(originalSettings);
+        }
+    }
+
+    private sealed class ShadeReportingPlatformSettings : DefaultPlatformSettings
+    {
+        public override PlatformColorValues GetColorValues() => new()
+        {
+            AccentColor1 = Colors.Crimson,
+            AccentColorDark1 = Colors.DarkRed,
+            AccentColorDark2 = Colors.Maroon,
+            AccentColorDark3 = Colors.Brown,
+            AccentColorLight1 = Colors.Salmon,
+            AccentColorLight2 = Colors.LightSalmon,
+            AccentColorLight3 = Colors.MistyRose,
+        };
+    }
+
     [AvaloniaFact]
     public void Legacy_palette_colors_derive_fluent2_tokens()
     {