fix(scrollviewer): [iOS] Ensure ScrollViewer.ViewportWidth is updated correctly

Accounts for asynchronous arrange pass on iOS.

Author: davidjohnoliver

src/Uno.UI/UI/Xaml/Controls/ScrollContentPresenter/NativeScrollContentPresenter.iOS.cs

@@ -33,6 +33,12 @@ partial class NativeScrollContentPresenter : UIScrollView, DependencyObject
 		internal NativeScrollContentPresenter(ScrollViewer scroller) : this()
 		{
 			_scrollViewer = new WeakReference<ScrollViewer>(scroller);
+
+			// Because the arrange pass is asynchronous on iOS, this is required for the ScrollViewer to get up-to-date viewport dimensions
+			SizeChanged += (_, __) =>
+			{
+				GetParentScrollViewer()?.UpdateDimensionProperties();
+			};
 		}
 
 		public NativeScrollContentPresenter()
@@ -61,7 +67,7 @@ private void OnScrolled(object sender, EventArgs e)
 
 		private void InvokeOnScroll()
 		{
-			var scroller = _scrollViewer.TryGetTarget(out var s) ? s : TemplatedParent as ScrollViewer;
+			var scroller = GetParentScrollViewer();
 			if (scroller is null)
 			{
 				return;
@@ -74,6 +80,8 @@ private void InvokeOnScroll()
 			scroller.OnScrollInternal(clampedOffset.X, clampedOffset.Y, isIntermediate: _isInAnimatedScroll);
 		}
 
+		private ScrollViewer GetParentScrollViewer() => _scrollViewer.TryGetTarget(out var s) ? s : TemplatedParent as ScrollViewer;
+
 		// Called when user starts dragging
 		private void OnDraggingStarted(object sender, EventArgs e)
 		{

src/Uno.UI/UI/Xaml/Controls/ScrollViewer/ScrollViewer.cs

@@ -652,7 +652,12 @@ internal override void OnLayoutUpdated()
 			UpdateZoomedContentAlignment();
 		}
 
-		private void UpdateDimensionProperties()
+#if __IOS__
+		internal
+#else
+		private
+#endif
+			void UpdateDimensionProperties()
 		{
 			if (this.Log().IsEnabled(LogLevel.Debug)
 				&& (ActualHeight != ViewportHeight || ActualWidth != ViewportWidth)
@@ -858,7 +863,7 @@ void IFrameworkTemplatePoolAware.OnTemplateRecycled()
 			}
 		}
 
-		#region Content and TemplatedParent forwarding to the ScrollContentPresenter
+#region Content and TemplatedParent forwarding to the ScrollContentPresenter
 		protected override void OnContentChanged(object oldValue, object newValue)
 		{
 			base.OnContentChanged(oldValue, newValue);
@@ -943,9 +948,9 @@ private void ClearContentTemplatedParent(object oldContent)
 				provider.Store.ClearValue(provider.Store.TemplatedParentProperty, DependencyPropertyValuePrecedences.Local);
 			}
 		}
-		#endregion
+#endregion
 
-		#region Managed scroll bars support
+#region Managed scroll bars support
 		private bool _isTemplateApplied;
 		private ScrollBar? _verticalScrollbar;
 		private ScrollBar? _horizontalScrollbar;
@@ -1100,7 +1105,7 @@ private void OnHorizontalScrollBarScrolled(object sender, ScrollEventArgs e)
 
 			ChangeViewScroll(e.NewValue, null, disableAnimation: immediate);
 		}
-		#endregion
+#endregion
 
 		// Presenter to Control, i.e. OnPresenterScrolled
 		internal void OnScrollInternal(double horizontalOffset, double verticalOffset, bool isIntermediate)
@@ -1214,7 +1219,7 @@ public bool ChangeView(double? horizontalOffset, double? verticalOffset, float?
 		partial void ChangeViewScroll(double? horizontalOffset, double? verticalOffset, bool disableAnimation);
 		partial void ChangeViewZoom(float zoomFactor, bool disableAnimation);
 
-		#region Scroll indicators visual states (Managed scroll bars only)
+#region Scroll indicators visual states (Managed scroll bars only)
 		private DispatcherQueueTimer? _indicatorResetTimer;
 		private string? _indicatorState;
 
@@ -1293,6 +1298,6 @@ private void HideScrollBarSeparator(object sender, PointerRoutedEventArgs e) //
 				VisualStateManager.GoToState(this, VisualStates.ScrollBarsSeparator.Collapsed, true);
 			}
 		}
-		#endregion
+#endregion
 	}
 }