Merge pull request #3750 from AvaloniaUI/fixes/2755-brush-opacity

Invalidate brush when Opacity changes.

src/Avalonia.Visuals/Media/Brush.cs

@@ -19,6 +19,11 @@ namespace Avalonia.Media
         /// <inheritdoc/>
         public event EventHandler Invalidated;
 
+        static Brush()
+        {
+            AffectsRender<Brush>(OpacityProperty);
+        }
+
         /// <summary>
         /// Gets or sets the opacity of the brush.
         /// </summary>

tests/Avalonia.Visuals.UnitTests/Media/BrushTests.cs

@@ -77,5 +77,17 @@ namespace Avalonia.Visuals.UnitTests.Media
         {
             Assert.Throws<FormatException>(() => Brush.Parse("#ff808g80"));
         }
+
+        [Fact]
+        public void Changing_Opacity_Raises_Invalidated()
+        {
+            var target = new SolidColorBrush();
+            var raised = false;
+
+            target.Invalidated += (s, e) => raised = true;
+            target.Opacity = 0.5;
+
+            Assert.True(raised);
+        }
     }
 }