fixes the overlay popup glitch where it renders at 0,0 before applying the offet.

use the media context api to sync with layout and render.

src/Avalonia.Base/Media/MediaContext.cs

@@ -212,7 +212,12 @@ internal partial class MediaContext : ICompositorScheduler
         }
         while (count > 0);
     }
-    
+
+    /// <summary>
+    /// Executes the <param name="callback">callback</param> in the layout pass that will occur
+    /// immediately before the next rendered frame.
+    /// </summary>
+    /// <param name="callback">Code to execute.</param>
     public void BeginInvokeOnRender(Action callback)
     {
         if (_invokeOnRenderCallbacks == null)
@@ -224,4 +229,4 @@ internal partial class MediaContext : ICompositorScheduler
         if (!_isRendering) 
             ScheduleRender(true);
     }
-}
+}

src/Avalonia.Controls/Primitives/OverlayPopupHost.cs

@@ -120,11 +120,11 @@ namespace Avalonia.Controls.Primitives
         void IManagedPopupPositionerPopup.MoveAndResize(Point devicePoint, Size virtualSize)
         {
             _lastRequestedPosition = devicePoint;
-            Dispatcher.UIThread.Post(() =>
+            MediaContext.Instance.BeginInvokeOnRender(() =>
             {
-                OverlayLayer.SetLeft(this, _lastRequestedPosition.X);
-                OverlayLayer.SetTop(this, _lastRequestedPosition.Y);
-            }, DispatcherPriority.Render);
+                Canvas.SetLeft(this, _lastRequestedPosition.X);
+                Canvas.SetTop(this, _lastRequestedPosition.Y);
+            });
         }
 
         double IManagedPopupPositionerPopup.Scaling => 1;