fix(textbox): textbox vertical scrolling not following the caret

src/Uno.UI/UI/Xaml/Controls/TextBox/TextBox.skia.cs

@@ -383,7 +383,7 @@ private void UpdateScrolling()
 
 			// TODO: we are sometimes horizontally overscrolling, but it's more visually pleasant that underscrolling as we want the caret to be fully showing.
 			var newHorizontalOffset = horizontalOffset.AtMost(rect.Left).AtLeast(Math.Ceiling(rect.Left - sv.ViewportWidth + InlineCollection.CaretThickness));
-			var newVerticalOffset = verticalOffset.AtMost(rect.Top).AtLeast(rect.Top - sv.ViewportWidth);
+			var newVerticalOffset = verticalOffset.AtMost(rect.Top).AtLeast(rect.Top - sv.ViewportHeight);
 
 			sv.ChangeView(newHorizontalOffset, newVerticalOffset, null);
 		}