fix(DependencyObject): Fix possible infinite loop on ancestor lookup

src/Uno.UI.Tests/DependencyProperty/Given_DependencyObject.cs

@@ -122,6 +122,21 @@ public void Should_Have_Bindable_Attribute()
 			Assert.AreEqual(1, typeof(MyObject).GetCustomAttributes(typeof(BindableAttribute), true).Length);
 		}
 
+		[TestMethod]
+		public void When_LayoutLoop()
+		{
+			var SUT = new ContentControl();
+			var inner1 = new Grid();
+			var inner2 = new Grid();
+
+			SUT.Content = inner1;
+			inner1.Children.Add(inner2);
+			inner2.Children.Add(SUT);
+
+			// No exception should be raised for this test, until
+			// Children.Add validates for cycles.
+		}
+
 		public partial class MyObject : DependencyObject
 		{
 			public MyObject(int value)
@@ -145,7 +160,6 @@ public override bool Equals(object obj)
 
 			public override int GetHashCode() => Value.GetHashCode();
 		}
-
 	}
 
 	public partial class MyProvider : DependencyObject

src/Uno.UI/UI/Xaml/DependencyObjectStore.cs

@@ -1572,7 +1572,10 @@ private static bool IsAncestor(DependencyObject? instance, AncestorsDictionary m
 
 			if (ancestor != null && !map.TryGetValue(ancestor, out isAncestor))
 			{
-				while (instance != null)
+				// Max iterations for ancestor lookup
+				var iterations = 1000;
+
+				while (instance != null && iterations-- > 0)
 				{
 #if DEBUG
 					var prevInstance = instance;
@@ -1584,7 +1587,6 @@ private static bool IsAncestor(DependencyObject? instance, AncestorsDictionary m
 					{
 						if (!hashSet.Contains(instance!))
 						{
-							// Console.WriteLine($"Added other {(instance as FrameworkElement)?.Name}");
 							hashSet.Add(instance);
 						}
 						else
@@ -1597,8 +1599,28 @@ private static bool IsAncestor(DependencyObject? instance, AncestorsDictionary m
 					if (instance == ancestor)
 					{
 						isAncestor = true;
+						break;
+					}
+				}
+
+				if (iterations < 1)
+				{
+					var level =
+#if DEBUG
+						// Make layout cycles visible in debug builds, until Children.Add
+						// validates for cycles.
+						LogLevel.Error;
+#else
+						// Layout cycles are ignored in this function for release builds.
+						LogLevel.Debug;
+#endif
+
+					if (ancestor.Log().IsEnabled(level))
+					{
+						ancestor.Log().Log(level, $"Possible cycle detected, ignoring ancestor [{ancestor}]");
 					}
 
+					isAncestor = false;
 				}
 
 				map.Set(ancestor, isAncestor);