Fix failing popup tests.

src/Avalonia.Controls/Primitives/LightDismissOverlayLayer.cs

@@ -1,10 +1,41 @@
+using System;
 using System.Linq;
-using Avalonia.Rendering;
+using Avalonia.Controls.Templates;
+using Avalonia.Styling;
 using Avalonia.VisualTree;
 
+#nullable enable
+
 namespace Avalonia.Controls.Primitives
 {
+    /// <summary>
+    /// A layer that is used to dismiss a <see cref="Popup"/> when the user clicks outside.
+    /// </summary>
     public class LightDismissOverlayLayer : Border
     {
+        /// <summary>
+        /// Returns the light dismiss overlay for a specified visual.
+        /// </summary>
+        /// <param name="visual">The visual.</param>
+        /// <returns>The light dismiss overlay, or null if none found.</returns>
+        public static LightDismissOverlayLayer? GetLightDismissOverlayLayer(IVisual visual)
+        {
+            visual = visual ?? throw new ArgumentNullException(nameof(visual));
+
+            VisualLayerManager? manager;
+
+            if (visual is TopLevel topLevel)
+            {
+                manager = topLevel.GetTemplateChildren()
+                    .OfType<VisualLayerManager>()
+                    .FirstOrDefault();
+            }
+            else
+            {
+                manager = visual.FindAncestorOfType<VisualLayerManager>();
+            }
+
+            return manager?.LightDismissOverlayLayer;
+        }
     }
 }

src/Avalonia.Controls/Primitives/Popup.cs

@@ -384,8 +384,7 @@ namespace Avalonia.Controls.Primitives
 
             if (!StaysOpen)
             {
-                var layerManager = placementTarget.FindAncestorOfType<VisualLayerManager>();
+                var dismissLayer = LightDismissOverlayLayer.GetLightDismissOverlayLayer(placementTarget);
-                var dismissLayer = layerManager?.LightDismissOverlayLayer;
 
                 if (dismissLayer != null)
                 {

tests/Avalonia.Controls.UnitTests/Primitives/PopupTests.cs

@@ -392,7 +392,8 @@ namespace Avalonia.Controls.UnitTests.Primitives
                     ++raised;
                 };
 
-                window.RaiseEvent(press);
+                var lightDismissLayer = window.FindDescendantOfType<VisualLayerManager>().LightDismissOverlayLayer;
+                lightDismissLayer.RaiseEvent(press);
 
                 Assert.Equal(1, raised);
             }