perf: [Wasm] Register on ScrollContentPresenter pointer events only w…

…hen native

src/Uno.UI/UI/Xaml/Controls/ScrollContentPresenter/ScrollContentPresenter.wasm.cs

@@ -19,6 +19,7 @@ public partial class ScrollContentPresenter : ContentPresenter, IScrollContentPr
 	{
 		private ScrollBarVisibility _verticalScrollBarVisibility;
 		private ScrollBarVisibility _horizontalScrollBarVisibility;
+		private bool _eventsRegistered;
 
 		internal Size ScrollBarSize
 		{
@@ -32,13 +33,29 @@ internal Size ScrollBarSize
 
 		public ScrollContentPresenter()
 		{
-			PointerReleased += ScrollViewer_PointerReleased;
-			PointerPressed += ScrollViewer_PointerPressed;
-			PointerCanceled += ScrollContentPresenter_PointerCanceled;
-			PointerMoved += ScrollContentPresenter_PointerMoved;
-			PointerEntered += ScrollContentPresenter_PointerEntered;
-			PointerExited += ScrollContentPresenter_PointerExited;
-			PointerWheelChanged += ScrollContentPresenter_PointerWheelChanged;
+		}
+
+		private void TryRegisterEvents(ScrollBarVisibility visibility)
+		{
+
+			if (
+				!_eventsRegistered
+				&& (visibility == ScrollBarVisibility.Auto || visibility == ScrollBarVisibility.Visible))
+			{
+				// Those events are only needed when native scrollbars are used, in order to handle
+				// pointer events on the native scrolbars themselves. See HandlePointerEvent for
+				// more details.
+
+				_eventsRegistered = true;
+
+				PointerReleased += ScrollViewer_PointerReleased;
+				PointerPressed += ScrollViewer_PointerPressed;
+				PointerCanceled += ScrollContentPresenter_PointerCanceled;
+				PointerMoved += ScrollContentPresenter_PointerMoved;
+				PointerEntered += ScrollContentPresenter_PointerEntered;
+				PointerExited += ScrollContentPresenter_PointerExited;
+				PointerWheelChanged += ScrollContentPresenter_PointerWheelChanged;
+			}
 		}
 
 		private void ScrollContentPresenter_PointerWheelChanged(object sender, Input.PointerRoutedEventArgs e)
@@ -79,8 +96,8 @@ private void HandlePointerEvent(Input.PointerRoutedEventArgs e)
 				return;
 			}
 
-			// The events coming from the scrollbars are bubbled up
-			// to the parents, as those are not (yet) XAML elements.
+			// The events coming from the native scrollbars are bubbled up
+			// to the parents, as those are not XAML elements.
 			// This can cause issues for popups with scrollable content and
 			// light dismiss patterns.
 			var position = e.GetCurrentPoint(this).Position;
@@ -105,6 +122,8 @@ internal ScrollBarVisibility VerticalScrollBarVisibility
 				{
 					_verticalScrollBarVisibility = value;
 					SetClasses(VerticalVisibilityClasses, (int)value);
+
+					TryRegisterEvents(value);
 				}
 			}
 		}
@@ -120,6 +139,8 @@ internal ScrollBarVisibility HorizontalScrollBarVisibility
 				{
 					_horizontalScrollBarVisibility = value;
 					SetClasses(HorizontalVisibilityClasses, (int)value);
+
+					TryRegisterEvents(value);
 				}
 			}
 		}