Added failing test for #2755.

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);
+        }
     }
 }