test: add ColorPickerHelpers bitmap accuracy tests

Verify that CreateComponentBitmapAsync with isPerceptive=false produces
pixel output that reflects the actual HSV selection values. Covers all
three component types (Hue, Saturation, Value) with assertions derived
from HSV color math invariants.

Also adds InternalsVisibleTo and project reference to enable testing
the internal ColorPickerHelpers from Avalonia.Controls.UnitTests.

src/Avalonia.Controls.ColorPicker/Avalonia.Controls.ColorPicker.csproj

@@ -23,6 +23,7 @@
 
   <ItemGroup Label="InternalsVisibleTo">
     <InternalsVisibleTo Include="Avalonia.DesignerSupport, PublicKey=$(AvaloniaPublicKey)" />
+    <InternalsVisibleTo Include="Avalonia.Controls.UnitTests, PublicKey=$(AvaloniaPublicKey)" />
   </ItemGroup>
 
 </Project>

tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj

@@ -23,6 +23,7 @@
     <ProjectReference Include="..\..\src\Markup\Avalonia.Markup\Avalonia.Markup.csproj" />
     <ProjectReference Include="..\..\src\Avalonia.Base\Avalonia.Base.csproj" />
     <ProjectReference Include="..\..\src\Avalonia.Controls\Avalonia.Controls.csproj" />
+    <ProjectReference Include="..\..\src\Avalonia.Controls.ColorPicker\Avalonia.Controls.ColorPicker.csproj" />
     <ProjectReference Include="..\Avalonia.UnitTests\Avalonia.UnitTests.csproj" />
     <Compile Include="../Shared/ScopedSanityCheck.cs"/>
     <Service Include="{82a7f48d-3b50-4b1e-b82e-3ada8210c358}" />

tests/Avalonia.Controls.UnitTests/ColorPickerTests/ColorPickerHelpersTests.cs

@@ -0,0 +1,110 @@
+using System;
+using System.Threading.Tasks;
+using Avalonia.Collections.Pooled;
+using Avalonia.Controls.Primitives;
+using Avalonia.Layout;
+using Avalonia.Media;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests.ColorPickerTests;
+
+/// <summary>
+/// Tests that <see cref="ColorPickerHelpers.CreateComponentBitmapAsync"/> produces
+/// accurate pixel output when <c>isPerceptive=false</c> — the mode the ColorSpectrum's
+/// third-component slider should use.
+///
+/// Regression tests for https://github.com/AvaloniaUI/Avalonia/issues/20925
+/// </summary>
+public class ColorPickerHelpersTests : ScopedTestBase
+{
+    /// <summary>
+    /// When sweeping Hue at a fixed desaturated selection (S=0.2, V=0.5),
+    /// every pixel's brightest channel should be bounded by V — not boosted to 255.
+    /// </summary>
+    [Fact]
+    public async Task Hue_Slider_Bitmap_Reflects_Actual_Saturation_And_Value()
+    {
+        var selection = new HsvColor(alpha: 1.0, hue: 180, saturation: 0.2, value: 0.5);
+
+        using var pixels = await GenerateSliderBitmap(ColorComponent.Component1, selection, height: 360);
+
+        // Sample mid-strip. In HSV, max RGB channel = V for any hue.
+        var (r, g, b) = ReadPixel(pixels, row: 180);
+        var maxChannel = Math.Max(r, Math.Max(g, b));
+        double expectedMax = selection.V * 255;
+
+        Assert.InRange(maxChannel, expectedMax - 10, expectedMax + 10);
+    }
+
+    /// <summary>
+    /// When sweeping Saturation, the brightest channel at full saturation is bounded
+    /// by the current Value — not forced to 255 by perceptive V=1.0 override.
+    /// </summary>
+    [Theory]
+    [InlineData(0.1)]
+    [InlineData(0.5)]
+    [InlineData(0.9)]
+    public async Task Saturation_Slider_Bitmap_Respects_Current_Value(double currentValue)
+    {
+        var selection = new HsvColor(alpha: 1.0, hue: 200, saturation: 0.8, value: currentValue);
+
+        using var pixels = await GenerateSliderBitmap(ColorComponent.Component2, selection);
+
+        // Row 0 = highest saturation (bitmap sweeps high→low top-to-bottom).
+        var (r, g, b) = ReadPixel(pixels, row: 0);
+        var maxChannel = Math.Max(r, Math.Max(g, b));
+        double expectedMax = currentValue * 255;
+
+        Assert.InRange(maxChannel, expectedMax - 20, expectedMax + 20);
+    }
+
+    /// <summary>
+    /// When sweeping Value at low Saturation (S=0.1), the top pixel should be
+    /// near-grey (low chroma) — not vivid from a perceptive S=1.0 override.
+    /// </summary>
+    [Fact]
+    public async Task Value_Slider_Bitmap_Respects_Current_Saturation()
+    {
+        var selection = new HsvColor(alpha: 1.0, hue: 30, saturation: 0.1, value: 0.8);
+
+        using var pixels = await GenerateSliderBitmap(ColorComponent.Component3, selection);
+
+        // Row 0 = highest Value. In HSV, chroma = V * S * 255 ≤ S * 255.
+        var (r, g, b) = ReadPixel(pixels, row: 0);
+        int chroma = Math.Max(r, Math.Max(g, b)) - Math.Min(r, Math.Min(g, b));
+
+        // At S=0.1 the max possible chroma is 25.5 — near-grey for any V.
+        // With perceptive S=1.0 override, chroma would be ~252.
+        Assert.InRange(chroma, 0, selection.S * 255 + 10);
+    }
+
+    private static async Task<PooledList<byte>> GenerateSliderBitmap(
+        ColorComponent component,
+        HsvColor baseColor,
+        int height = 100)
+    {
+        const int width = 1;
+        var buffer = new PooledList<byte>(width * height * 4, ClearMode.Never, sizeToCapacity: true);
+
+        await ColorPickerHelpers.CreateComponentBitmapAsync(
+            buffer, width, height,
+            Orientation.Vertical,
+            ColorModel.Hsva,
+            component,
+            baseColor,
+            isAlphaVisible: false,
+            isPerceptive: false);
+
+        return buffer;
+    }
+
+    /// <summary>
+    /// Reads a pixel from a 1-pixel-wide vertical BGRA bitmap at the given row.
+    /// </summary>
+    private static (byte R, byte G, byte B) ReadPixel(PooledList<byte> bgraData, int row)
+    {
+        int offset = row * 4;
+        return (R: bgraData[offset + 2], G: bgraData[offset + 1], B: bgraData[offset]);
+    }
+}