Move Screens property to WindowBase since they are in IWindowBaseImpl anyway

src/Avalonia.Controls/Primitives/PopupRoot.cs

@@ -80,7 +80,7 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         public void SnapInsideScreenEdges()
         {
-            var screen = Application.Current.MainWindow?.Screens.ScreenFromPoint(Position);
+            var screen = (VisualRoot as WindowBase)?.Screens?.ScreenFromPoint(Position);
 
             if (screen != null)
             {

src/Avalonia.Controls/Screens.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using Avalonia.Platform;
 using Avalonia.Utilities;
@@ -11,7 +12,7 @@ namespace Avalonia.Controls
         private readonly IScreenImpl _iScreenImpl;
 
         public int ScreenCount => _iScreenImpl.ScreenCount;
-        public IReadOnlyList<Screen> All => _iScreenImpl?.AllScreens;
+        public IReadOnlyList<Screen> All => _iScreenImpl?.AllScreens ?? Array.Empty<Screen>();
         public Screen Primary => All.FirstOrDefault(x => x.Primary);
 
         public Screens(IScreenImpl iScreenImpl)

src/Avalonia.Controls/Window.cs

@@ -140,7 +140,6 @@ namespace Avalonia.Controls
             impl.Closing = HandleClosing;
             impl.WindowStateChanged = HandleWindowStateChanged;
             _maxPlatformClientSize = PlatformImpl?.MaxClientSize ?? default(Size);
-            Screens = new Screens(PlatformImpl?.Screen);
         }
 
         /// <inheritdoc/>
@@ -157,8 +156,6 @@ namespace Avalonia.Controls
             remove { _nameScope.Unregistered -= value; }
         }
 
-        public Screens Screens { get; private set; }
-
         /// <summary>
         /// Gets the platform-specific window implementation.
         /// </summary>

src/Avalonia.Controls/WindowBase.cs

@@ -63,6 +63,7 @@ namespace Avalonia.Controls
 
         public WindowBase(IWindowBaseImpl impl, IAvaloniaDependencyResolver dependencyResolver) : base(impl, dependencyResolver)
         {
+            Screens = new Screens(PlatformImpl?.Screen);
             impl.Activated = HandleActivated;
             impl.Deactivated = HandleDeactivated;
             impl.PositionChanged = HandlePositionChanged;
@@ -108,6 +109,8 @@ namespace Avalonia.Controls
                     impl.Position = value;
             }
         }
+        
+        public Screens Screens { get; private set; }
 
         /// <summary>
         /// Whether an auto-size operation is in progress.