[macOS] Visual glitch when exiting the full screen with ScrollViewer (x…

…amarin#8086) fixes xamarin#4854

* added test for issue4854

* fix issue4854

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue4854.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Linq.Expressions;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 4854, "[macOS] Visual glitch when exiting the full screen with ScrollViewer", PlatformAffected.macOS)]
+	public class Issue4854 : TestContentPage
+	{
+
+		protected override void Init()
+		{
+			var gMain = new Grid { BackgroundColor = Color.LightBlue };
+			gMain.RowDefinitions.Add(new RowDefinition { Height = GridLength.Star });
+			gMain.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
+
+			var label = new Label
+			{
+				Text = "Enter full screen and exit and see the artifacts on the screen.",
+				FontSize = 14
+			};
+			var sl = new StackLayout { HorizontalOptions = LayoutOptions.Center, WidthRequest = 300, Padding = new Thickness(15) };
+			sl.Children.Add(label);
+			gMain.Children.Add(sl);
+
+			var button = new Button { Text = "Test", BackgroundColor = Color.Gray, HorizontalOptions = LayoutOptions.Center };
+			var g = new Grid { BackgroundColor = Color.LightGray, Padding = new Thickness(20) };
+			g.Children.Add(button);
+			Grid.SetRow(g, 1);
+			gMain.Children.Add(g);
+
+			Content = new ScrollView { Content = gMain, BackgroundColor = Color.LightGreen };
+		}
+
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -1123,6 +1123,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Issue4356.cs">
       <DependentUpon>Issue4356.xaml</DependentUpon>
     </Compile>
+	<Compile Include="$(MSBuildThisFileDirectory)Issue4854.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue5951.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Github6021.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue5132.cs" />

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

@@ -255,7 +255,7 @@ private bool ResetNativeNonScroll( )
 			if (ContentView == null || ScrollView == null || ScrollView.Content == null)
 				return false;
 
-			if (Math.Abs(ScrollView.ScrollY) < 0.001 && Math.Abs(ScrollView.ScrollX) < 0.001 && ScrollView.Content.Height > ScrollView.Height)
+			if (Math.Abs(ScrollView.ScrollY) < 0.001 && Math.Abs(ScrollView.ScrollX) < 0.001 && ScrollView.Content.Height >= ScrollView.Height)
 			{
 				ContentView.ScrollToPoint(new CoreGraphics.CGPoint(0, 0));
 				return true;
@@ -270,12 +270,16 @@ void UpdateScrollPosition()
 			if (ScrollView == null)
 				return;
 
-			if (ScrollView.ContentSize.Height >= ScrollView.Height)
+			var height = ScrollView.Height;
+			var contentHeightOverflow = ScrollView.ContentSize.Height - height;
+			if (contentHeightOverflow >= 0)
 			{
-				CoreGraphics.CGPoint location = ContentView.DocumentVisibleRect().Location;
-
-				if (location.Y > -1 && ScrollView.Height >= 0)
-					ScrollView.SetScrolledPosition(Math.Max(0, location.X), Math.Max(0, ScrollView.ContentSize.Height - ScrollView.Height - location.Y));
+				if (height >= 0)
+				{
+					var location = ContentView.DocumentVisibleRect().Location;
+					if (location.Y > -1)
+						ScrollView.SetScrolledPosition(Math.Max(0, location.X), Math.Max(0, contentHeightOverflow - location.Y));
+				}
 			}
 			else
 				ResetNativeNonScroll();