Ensure that the z-index sort also preserves original add order (#7270)

Fixes #7122

src/Core/src/Handlers/Layout/LayoutExtensions.cs

@@ -5,26 +5,49 @@ namespace Microsoft.Maui.Handlers
 {
 	internal static class LayoutExtensions
 	{
-		class ZIndexComparer : IComparer<IView>
+		record ViewAndIndex(IView View, int Index);
+
+		class ZIndexComparer : IComparer<ViewAndIndex>
 		{
-			public int Compare(IView? x, IView? y)
+			public int Compare(ViewAndIndex? x, ViewAndIndex? y)
 			{
-				if (x == null || y == null)
+				if(x == null || y == null)
 				{
 					return 0;
 				}
 
-				return x.ZIndex.CompareTo(y.ZIndex);
+				var zIndexCompare = x.View.ZIndex.CompareTo(y.View.ZIndex);
+
+				if (zIndexCompare == 0)
+				{
+					return x.Index.CompareTo(y.Index);
+				}
+
+				return zIndexCompare;
 			}
 		}
 
 		static ZIndexComparer s_comparer = new();
 
 		public static IView[] OrderByZIndex(this ILayout layout)
 		{
-			var ordered = new IView[layout.Count];
-			layout.CopyTo(ordered, 0);
-			Array.Sort(ordered, s_comparer);
+			var count = layout.Count;	
+			var indexedViews = new ViewAndIndex[count];
+
+			for (int n = 0; n < count; n++)
+			{
+				indexedViews[n] = new ViewAndIndex(layout[n], n);
+			}
+
+			Array.Sort(indexedViews, s_comparer);
+
+			var ordered = new IView[count];
+
+			for (int n = 0; n < count; n++)
+			{
+				ordered[n] = indexedViews[n].View;
+			}
+
 			return ordered;
 		}
 

src/Core/tests/UnitTests/Layouts/ZIndexTests.cs

@@ -243,5 +243,37 @@ public void ZIndexUpdatePreservesAddOrderForEqualZIndexes()
 			Assert.Equal(view2, zordered[2]);
 			Assert.Equal(view3, zordered[3]);
 		}
+
+		[Fact]
+		public void ZIndexUpdatePreservesAddOrderLotsOfEqualZIndexes()
+		{
+			// This tests the same thing as ZIndexUpdatePreservesAddOrderForEqualZIndexes,
+			// but for more views - since the sorting algorithm can change when the arrays 
+			// are larger, we were running into situations where layouts with more controls
+			// were _not_ preserving the Add() order when sorting by z-index.
+
+			var layout = new FakeLayout();
+
+			int views = 100;
+			int iterations = 10;
+
+			var toAdd = new IView[views];
+
+			for (int n = 0; n < views; n++)
+			{
+				toAdd[n] = CreateTestView(zIndex: 0);
+				layout.Add(toAdd[n]);
+			}
+
+			for (int i = 0; i < iterations; i++)
+			{
+				var zordered = layout.OrderByZIndex();
+
+				for (int n = 0; n < zordered.Length; n++)
+				{
+					Assert.Equal(toAdd[n], zordered[n]);
+				}
+			}
+		}
 	}
 }