fix: Update Shape.Fill when LinearGradientBrush gradient stops change

Change GradientStopCollection to inherit from DependencyObjectCollection, to invalidate parent view when collection is changed.

src/SamplesApp/SamplesApp.UITests/Windows_UI_Xaml_Media/GradientBrushTests/LinearGradientBrush_Tests.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using NUnit.Framework;
+using SamplesApp.UITests.TestFramework;
+using Uno.UITest.Helpers;
+
+namespace SamplesApp.UITests.Windows_UI_Xaml_Media.GradientBrushTests
+{
+	[TestFixture]
+	public class LinearGradientBrush_Tests : SampleControlUITestBase
+	{
+		[Test]
+		[AutoRetry]
+		public void When_GradientStops_Changed()
+		{
+			Run("UITests.Windows_UI_Xaml_Media.GradientBrushTests.LinearGradientBrush_Change_Stops");
+
+			var rectangle = _app.Marked("GradientBrushRectangle");
+
+			_app.WaitForElement(rectangle);
+
+			var screenRect = _app.GetRect(rectangle);
+
+			var before = TakeScreenshot("Before");
+
+			_app.FastTap("ChangeBrushButton");
+
+			_app.WaitForText("StatusTextBlock", "Changed");
+
+			var after = TakeScreenshot("After");
+
+			ImageAssert.AreNotEqual(before, after, screenRect);
+		}
+	}
+}

src/SamplesApp/UITests.Shared/UITests.Shared.projitems

@@ -2837,6 +2837,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media\GradientBrushTests\LinearGradientBrush_Change_Stops.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
     <Page Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media\ImageBrushTests\ImageBrush_WriteableBitmap.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
@@ -4945,6 +4949,9 @@
       <DependentUpon>ClosedFigurePage.xaml</DependentUpon>
     </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media\GradientBrushTests\GradientsPage.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media\GradientBrushTests\LinearGradientBrush_Change_Stops.xaml.cs">
+      <DependentUpon>LinearGradientBrush_Change_Stops.xaml</DependentUpon>
+    </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media\ImageBrushTests\ImageBrush_WriteableBitmap.xaml.cs">
       <DependentUpon>ImageBrush_WriteableBitmap.xaml</DependentUpon>
     </Compile>

src/SamplesApp/UITests.Shared/Windows_UI_Xaml_Media/GradientBrushTests/LinearGradientBrush_Change_Stops.xaml

@@ -0,0 +1,49 @@
+<UserControl
+    x:Class="UITests.Windows_UI_Xaml_Media.GradientBrushTests.LinearGradientBrush_Change_Stops"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:UITests.Windows_UI_Xaml_Media.GradientBrushTests"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d"
+    d:DesignHeight="300"
+    d:DesignWidth="400">
+
+	<UserControl.Resources>
+		<Color x:Key="UnoBlueColor">#FF229DFC</Color>
+		<Color x:Key="UnoPurpleColor">#FF7A69F5</Color>
+		<Color x:Key="UnoGreenColor">#FF6CE5AE</Color>
+		<Color x:Key="UnoRedColor">#FFF65678</Color>
+	</UserControl.Resources>
+
+	<StackPanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+		<Rectangle x:Name="GradientBrushRectangle"
+				Height="100"
+				Width="150"
+				HorizontalAlignment="Left">
+			<Rectangle.Fill>
+				<LinearGradientBrush x:Name="UnoGradientBrush"
+									 StartPoint="0,0"
+									 EndPoint="1,1">
+					<GradientStop Color="{StaticResource UnoGreenColor}"
+								  Offset="0.0" />
+					<GradientStop Color="{StaticResource UnoGreenColor}"
+								  Offset="0.1" />
+					<GradientStop Color="{StaticResource UnoBlueColor}"
+								  Offset="0.40" />
+					<GradientStop Color="{StaticResource UnoPurpleColor}"
+								  Offset="0.60" />
+					<GradientStop Color="{StaticResource UnoRedColor}"
+								  Offset="0.9" />
+					<GradientStop Color="{StaticResource UnoRedColor}"
+								  Offset="1" />
+				</LinearGradientBrush>
+			</Rectangle.Fill>
+		</Rectangle>
+		<Button x:Name="ChangeBrushButton"
+				Content="Change gradient stops"
+				Click="ChangeBrushButton_Click" />
+		<TextBlock x:Name="StatusTextBlock"
+				   Text="Not started" />
+	</StackPanel>
+</UserControl>

src/SamplesApp/UITests.Shared/Windows_UI_Xaml_Media/GradientBrushTests/LinearGradientBrush_Change_Stops.xaml.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Uno.UI.Samples.Controls;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Controls.Primitives;
+using Windows.UI.Xaml.Data;
+using Windows.UI.Xaml.Input;
+using Windows.UI.Xaml.Media;
+using Windows.UI.Xaml.Navigation;
+
+// The User Control item template is documented at https://go.microsoft.com/fwlink/?LinkId=234236
+
+namespace UITests.Windows_UI_Xaml_Media.GradientBrushTests
+{
+	[Sample("GradientBrush")]
+	public sealed partial class LinearGradientBrush_Change_Stops : UserControl
+	{
+		public LinearGradientBrush_Change_Stops()
+		{
+			this.InitializeComponent();
+		}
+
+		private void ChangeBrushButton_Click(object sender, RoutedEventArgs e)
+		{
+			UnoGradientBrush.GradientStops.RemoveAt(2);
+			UnoGradientBrush.GradientStops.RemoveAt(2);
+			StatusTextBlock.Text = "Changed";
+		}
+	}
+}

src/Uno.UI/UI/Xaml/Media/GradientBrush.cs

@@ -24,7 +24,10 @@ public Color FallbackColor
 			"GradientStops",
 			typeof(GradientStopCollection),
 			typeof(GradientBrush),
-			new FrameworkPropertyMetadata(null)
+				new FrameworkPropertyMetadata(
+					defaultValue: null,
+					options: FrameworkPropertyMetadataOptions.ValueInheritsDataContext | FrameworkPropertyMetadataOptions.LogicalChild | FrameworkPropertyMetadataOptions.AffectsArrange
+				)
 		);
 
 		public GradientStopCollection GradientStops

src/Uno.UI/UI/Xaml/Media/GradientStopCollection.cs

@@ -4,7 +4,12 @@
 
 namespace Windows.UI.Xaml.Media
 {
-	public sealed partial class GradientStopCollection : List<GradientStop>, IList<GradientStop>, IEnumerable<GradientStop>
+	public sealed partial class GradientStopCollection : DependencyObjectCollection<GradientStop>, IList<GradientStop>, IEnumerable<GradientStop>
 	{
+		private protected override void OnCollectionChanged()
+		{
+			base.OnCollectionChanged();
+			this.InvalidateArrange();
+		}
 	}
 }

src/Uno.UI/UI/Xaml/Shapes/Shape.cs

@@ -52,7 +52,7 @@ public Brush Fill
 				options: FrameworkPropertyMetadataOptions.ValueInheritsDataContext,
 				propertyChangedCallback: (s, e) => ((Shape)s).OnFillChanged((Brush)e.NewValue)
 #else
-				options: FrameworkPropertyMetadataOptions.ValueInheritsDataContext | FrameworkPropertyMetadataOptions.AffectsArrange,
+				options: FrameworkPropertyMetadataOptions.ValueInheritsDataContext | FrameworkPropertyMetadataOptions.LogicalChild | FrameworkPropertyMetadataOptions.AffectsArrange,
 				propertyChangedCallback: (s, e) => ((Shape)s)._brushChanged.Disposable = Brush.AssignAndObserveBrush((Brush)e.NewValue, _ => s.InvalidateArrange())
 #endif
 			)