Fix "ContextMenu ignores styles on first open"

src/Avalonia.Controls/ContextMenu.cs

@@ -351,6 +351,11 @@ namespace Avalonia.Controls
                 ? PlacementMode.Bottom
                 : PlacementMode;
 
+            //Position of the line below is really important. 
+            //All styles are being applied only when control has logical parent.
+            //Line below will add ContextMenu as child to the Popup and this will trigger styles and they would be applied.
+            //If you will move line below somewhere else it may cause that ContextMenu will behave differently from what you are expecting.
+            _popup.Child = this;
             _popup.PlacementTarget = placementTarget;
             _popup.HorizontalOffset = HorizontalOffset;
             _popup.VerticalOffset = VerticalOffset;
@@ -359,7 +364,6 @@ namespace Avalonia.Controls
             _popup.PlacementGravity = PlacementGravity;
             _popup.PlacementRect = PlacementRect;
             _popup.WindowManagerAddShadowHint = WindowManagerAddShadowHint;
-            _popup.Child = this;
             IsOpen = true;
             _popup.IsOpen = true;