[macOS] Adds Scrollbars to ListViews and ScrollViews (#740)

Xamarin.Forms.Platform.MacOS/Renderers/ListViewRenderer.cs

@@ -106,7 +106,8 @@ namespace Xamarin.Forms.Platform.MacOS
 					var scroller = new NSScrollView
 					{
 						AutoresizingMask = NSViewResizingMask.HeightSizable | NSViewResizingMask.WidthSizable,
-						DocumentView = _table = CreateNSTableView(e.NewElement)
+						DocumentView = _table = CreateNSTableView(e.NewElement),
+						HasVerticalScroller = true
 					};
 					SetNativeControl(scroller);
 				}

Xamarin.Forms.Platform.MacOS/Renderers/ScrollViewRenderer.cs

@@ -23,6 +23,7 @@ namespace Xamarin.Forms.Platform.MacOS
 			ContentView.PostsBoundsChangedNotifications = true;
 			NSNotificationCenter.DefaultCenter.AddObserver(this, new Selector(nameof(UpdateScrollPosition)),
 				BoundsChangedNotification, ContentView);
+			HasVerticalScroller = true;
 		}
 
 		IScrollViewController Controller => Element as IScrollViewController;