Fixed incorrect opacity stack handling (#20730)

Author: kekekeks

src/Avalonia.Base/Rendering/Composition/Server/ServerCompositionVisual/ServerCompositionVisual.Render.cs

@@ -94,7 +94,8 @@ private bool HandlePreGraphTransformClipOpacity(ServerCompositionVisual visual)
 
             if (visual.Opacity != 1)
             {
-                _opacityStack.Push(effectiveOpacity);
+                _opacityStack.Push(_opacity);
+                _opacity = effectiveOpacity;
                 _canvas.PushOpacity(visual.Opacity, visual._transformedSubTreeBounds.Value.ToRect());
             }
 

tests/Avalonia.RenderTests/BugRepros.cs

@@ -0,0 +1,55 @@
+using Avalonia.Controls;
+using Avalonia.Controls.Shapes;
+using Avalonia.Layout;
+using Avalonia.Media;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Avalonia.Skia.RenderTests;
+
+public class BugRepros() : TestBase(nameof(BugRepros))
+{
+    [Fact]
+    public async Task Sibling_Visuals_With_Opacity_Should_Not_Affect_Each_Other()
+    {
+        var brushes = new IBrush[]
+        {
+            Brushes.Red,
+            Brushes.Green,
+            Brushes.Blue,
+            Brushes.Yellow,
+            Brushes.Magenta,
+            Brushes.Cyan,
+            Brushes.Orange,
+            Brushes.Purple,
+            Brushes.Pink,
+            Brushes.Brown
+        };
+
+        var stackPanel = new StackPanel
+        {
+            Orientation = Orientation.Vertical,
+            Width = 300,
+            Height = 500,
+            Background = Brushes.White,
+        };
+
+        for (int i = 0; i < brushes.Length; i++)
+        {
+            var border = new Border
+            {
+                Width = 280,
+                Height = 40,
+                BorderThickness = new Thickness(2),
+                Margin = new Thickness(5),
+                Background = brushes[i],
+                Opacity = 0.3
+            };
+            stackPanel.Children.Add(border);
+        }
+
+        await RenderToFile(stackPanel);
+        CompareImages();
+    }
+}