[C] respect specificity while overriding DynRes (#24306)

fixes an unreported issue

src/Controls/src/Core/Element/Element.cs

@@ -749,7 +749,8 @@ internal void OnResourcesChanged(IEnumerable<KeyValuePair<string, object>> value
 		internal override void OnSetDynamicResource(BindableProperty property, string key, SetterSpecificity specificity)
 		{
 			base.OnSetDynamicResource(property, key, specificity);
-			DynamicResources[property] = (key, specificity);
+			if (!DynamicResources.TryGetValue(property, out var existing) || existing.Item2.CompareTo(specificity) < 0)
+				DynamicResources[property] = (key, specificity);
 			if (this.TryGetResource(key, out var value))
 				OnResourceChanged(property, value, specificity);
 		}

src/Controls/tests/Xaml.UnitTests/Issues/Unreported010.xaml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+                    x:Class="Microsoft.Maui.Controls.Xaml.UnitTests.Unreported010">
+    <ContentPage.Resources>
+        <Color x:Key="Foo">Blue</Color>
+        <Style TargetType="Button">
+            <Setter Property="BackgroundColor" Value="{DynamicResource Foo}"/>
+        </Style>
+    </ContentPage.Resources>
+
+    <VerticalStackLayout>
+        <Button x:Name="button0" Text="Hello World" BackgroundColor="{DynamicResource Foo}"/>
+    </VerticalStackLayout>
+</ContentPage>

src/Controls/tests/Xaml.UnitTests/Issues/Unreported010.xaml.cs

@@ -0,0 +1,47 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.Maui.ApplicationModel;
+using Microsoft.Maui.Graphics;
+using Microsoft.Maui.Controls.Core.UnitTests;
+using Microsoft.Maui.Dispatching;
+using Microsoft.Maui.UnitTests;
+using NUnit.Framework;
+
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public partial class Unreported010
+{
+    public Unreported010()
+    {
+        InitializeComponent();
+    }
+
+    public Unreported010(bool useCompiledXaml)
+    {
+        //this stub will be replaced at compile time
+    }
+
+    [TestFixture]
+    class Test
+    {
+        [SetUp]
+        public void Setup()
+        {
+            Application.SetCurrentApplication(new MockApplication());
+            DispatcherProvider.SetCurrent(new DispatcherProviderStub());
+        }
+
+        [TearDown] public void TearDown() => AppInfo.SetCurrent(null);
+
+        [Test]
+        public void LocalDynamicResources([Values(false, true)] bool useCompiledXaml)
+        {
+            var page = new Unreported010(useCompiledXaml);
+            Assert.That(page.button0.BackgroundColor, Is.EqualTo(Colors.Blue));
+            page.Resources["Foo"] = Colors.Red;
+            Assert.That(page.button0.BackgroundColor, Is.EqualTo(Colors.Red));
+        }
+    }
+
+
+}