Merge pull request #7187 from AvaloniaUI/fixes/catalina-red-window

osx: Fixes red flicker on Catalina when resizing

native/Avalonia.Native/src/OSX/window.mm

@@ -43,6 +43,7 @@ public:
         StandardContainer = [[AutoFitContentView new] initWithContent:View];
 
         Window = [[AvnWindow alloc] initWithParent:this];
+        [Window setContentView: StandardContainer];
         
         lastPositionSet.X = 100;
         lastPositionSet.Y = 100;
@@ -124,8 +125,6 @@ public:
             SetPosition(lastPositionSet);
             UpdateStyle();
             
-            [Window setContentView: StandardContainer];
-            
             [Window setTitle:_lastTitle];
             
             if(ShouldTakeFocusOnShow() && activate)
@@ -328,8 +327,8 @@ public:
                     BaseEvents->Resized(AvnSize{x,y}, reason);
                 }
                 
-                [StandardContainer setFrameSize:NSSize{x,y}];
-                [Window setContentSize:NSSize{x, y}];
+                [Window setContentSize:NSSize{x,y}];
+                [Window invalidateShadow];
             }
             @finally
             {