[macOS] Navigating back from a complex page is highly inefficient (#3406) fixes #3001

* fixes #3001 [macOS] Navigating back from a complex page is highly inefficient

* fixes UITEST compilation

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue3001.cs

@@ -0,0 +1,87 @@
+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, 3001, "[macOS] Navigating back from a complex page is highly inefficient", PlatformAffected.macOS)]
+	public class Issue3001 : TestContentPage
+	{
+		const string ButtonId = "ClearButton";
+		const string ReadyId = "ReadyLabel";
+
+		int _counter = 0;
+		int _level = 0;
+		const int maxLevel = 5;
+
+		public View BuildLevel()
+		{
+			if (_level == maxLevel)
+			{
+				_counter++;
+				return new Label { Text = _counter.ToString(), FontSize = 10 };
+			}
+
+			_level++;
+			var g1 = new Grid { RowSpacing = 0, ColumnSpacing = 0 };
+
+			var g2 = new Grid { RowSpacing = 0, ColumnSpacing = 0 };
+			g1.Children.Add(g2);
+
+			var g = new Grid { RowSpacing = 0, ColumnSpacing = 0 };
+			g2.Children.Add(g);
+
+			g.RowDefinitions.Add(new RowDefinition { Height = GridLength.Star });
+			g.RowDefinitions.Add(new RowDefinition { Height = GridLength.Star });
+			g.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Star });
+			g.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Star });
+
+			g.Children.Add(BuildLevel(), 0, 0);
+			g.Children.Add(BuildLevel(), 0, 1);
+			g.Children.Add(BuildLevel(), 1, 0);
+			g.Children.Add(BuildLevel(), 1, 1);
+
+			_level--;
+
+			return g1;
+		}
+
+		protected override void Init()
+		{
+			var sp = new StackLayout();
+			sp.Children.Add(new Button
+			{
+				Text = "Start",
+				AutomationId = ButtonId,
+				Command = new Command(() =>
+				{
+					Content = new Label
+					{
+						Text = "Ready",
+						AutomationId = ReadyId
+					};
+				})
+			});
+			sp.Children.Add(BuildLevel());
+			Content = sp;
+		}
+
+#if UITEST
+#if __MACOS__
+		[Test]
+		public void Issue3001Test()
+		{
+			RunningApp.WaitForElement(q => q.Marked(ButtonId));
+			RunningApp.Tap(q => q.Marked(ButtonId));
+			RunningApp.WaitForElement(q => q.Marked(ReadyId), timeout: TimeSpan.FromSeconds(5));
+		}
+#endif
+#endif
+	}
+}

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

@@ -251,6 +251,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Issue1556.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue1799.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue1931.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue3001.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue3271.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue3390.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue3000.cs" />

Xamarin.Forms.Platform.MacOS/Platform.cs

@@ -134,11 +134,15 @@ namespace Xamarin.Forms.Platform.MacOS
 
 		internal static void DisposeModelAndChildrenRenderers(Element view)
 		{
-			IVisualElementRenderer renderer;
 			foreach (VisualElement child in view.Descendants())
-				DisposeModelAndChildrenRenderers(child);
+				DisposeRenderer(child);
 
-			renderer = GetRenderer((VisualElement)view);
+			DisposeRenderer(view);
+		}
+
+		static void DisposeRenderer(Element view)
+		{
+			IVisualElementRenderer renderer = GetRenderer((VisualElement)view);
 			if (renderer?.ViewController?.ParentViewController != null)
 				renderer?.ViewController?.RemoveFromParentViewController();