Prevent overeager cache invalidation when the CollectionView height changes by very small amounts (#13738)

Fixes #13719

Author: hartez

Xamarin.Forms.Platform.iOS/CollectionView/ItemsViewLayout.cs

@@ -17,6 +17,8 @@ public abstract class ItemsViewLayout : UICollectionViewFlowLayout
 		CGSize _adjustmentSize1;
 		CGSize _currentSize;
 
+		const double ConstraintSizeTolerance = 0.00001;
+
 		Dictionary<object, CGSize> _cellSizeCache = new Dictionary<object, CGSize>();
 
 		public ItemsUpdatingScrollMode ItemsUpdatingScrollMode { get; set; }
@@ -88,7 +90,7 @@ protected virtual void HandlePropertyChanged(PropertyChangedEventArgs propertyCh
 
 		internal virtual void UpdateConstraints(CGSize size)
 		{
-			if (size == _currentSize)
+			if (!RequiresConstraintUpdate(size, _currentSize))
 			{
 				return;
 			}
@@ -588,5 +590,20 @@ internal void ClearCellSizeCache()
 		{
 			_cellSizeCache.Clear();
 		}
+
+		bool RequiresConstraintUpdate(CGSize newSize, CGSize current)
+		{
+			if (Math.Abs(newSize.Width - current.Width) > ConstraintSizeTolerance)
+			{
+				return true;
+			}
+
+			if (Math.Abs(newSize.Height - current.Height) > ConstraintSizeTolerance)
+			{
+				return true;
+			}
+
+			return false;
+		}
 	}
 }