fix(calendar): Fix scrolling issues on iOS (inc. allow ChangeView on …

…SV when not yet arranged)

src/Uno.UI/Microsoft/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs

@@ -480,6 +480,13 @@ private Size base_MeasureOverride(Size availableSize)
 			}
 
 			var viewport = GetLayoutViewport(availableSize);
+			if (Rows > 0 && Cols > 0)
+			{
+				// Uno: This SetViewportSize should be done in the CalendarPanel_Partial.ArrangeOverride of the Panel(not the 'base_'),
+				// but (due to invalid layouting event sequence in uno?) it would cause a second layout pass.
+				// Invoking it here makes sure that the ItemSize is valid for this measure pass.
+				_layoutStrategy.SetViewportSize(viewport.Size, out _);
+			}
 
 			_layoutStrategy.BeginMeasure();
 #if __ANDROID__ // TODO: IOS
@@ -488,6 +495,7 @@ private Size base_MeasureOverride(Size availableSize)
 			ShouldInterceptInvalidate = true;
 #endif
 			var index = -1;
+			var bottom = 0.0;
 			try
 			{
 				// Gets the index of the first element to render and the actual viewport to use
@@ -530,7 +538,9 @@ private Size base_MeasureOverride(Size availableSize)
 					var itemSize = _layoutStrategy.GetElementMeasureSize(ElementType.ItemContainer, index, renderWindow); // Note: It's actually the same for all items
 					var itemBounds = _layoutStrategy.GetElementBounds(ElementType.ItemContainer, index + StartIndex, itemSize, layout, renderWindow);
 
-					if (itemSize.Width < _minCellSize.Width && itemSize.Height < _minCellSize.Height || Cols == 0 || Rows == 0)
+					bottom = itemBounds.Bottom;
+
+					if ((itemSize.Width < _minCellSize.Width && itemSize.Height < _minCellSize.Height) || Cols == 0 || Rows == 0)
 					{
 						// We don't have any valid cell size yet (This measure pass has been caused by DetermineTheBiggestItemSize),
 						// so we stop right after having inserted the first child in the Children collection.
@@ -610,6 +620,12 @@ private Size base_MeasureOverride(Size availableSize)
 				default /* not used by CalendarLayoutStrategyImpl */,
 				out var desiredSize);
 
+			// When the StartIndex is greater than 0, the desiredSize might ignore the last line.
+			if (desiredSize.Height < bottom)
+			{
+				desiredSize.Height = bottom;
+			}
+
 			return desiredSize;
 		}
 
@@ -646,7 +662,7 @@ private static void OnEffectiveViewportChanged(FrameworkElement sender, Effectiv
 			{
 				that._effectiveViewport = args.EffectiveViewport;
 
-				if (that._host is null || that._layoutStrategy is null)
+				if (that._host is null || that._layoutStrategy is null || that.Cols == 0 || that.Rows == 0)
 				{
 					return;
 				}

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

@@ -27,8 +27,19 @@ partial class NativeScrollContentPresenter : UIScrollView, DependencyObject
 		/// </summary>
 		private bool _isInAnimatedScroll;
 
-		internal CGPoint UpperScrollLimit => (CGPoint)(ContentSize - Frame.Size);
 		CGPoint IUIScrollView.UpperScrollLimit => UpperScrollLimit;
+		internal CGPoint UpperScrollLimit
+		{
+			get
+			{
+				var extent = ContentSize;
+				var viewport = Frame.Size;
+
+				return new CGPoint(
+					Math.Max(0, extent.Width - viewport.Width),
+					Math.Max(0, extent.Height - viewport.Height));
+			}
+		}
 
 		internal NativeScrollContentPresenter(ScrollViewer scroller) : this()
 		{

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

@@ -57,24 +57,46 @@ private void SetScrollableContainer()
 			}
 		}
 
+		private (double? horizontal, double? vertical, bool disableAnimation)? _pendingChangeView;
+
+		protected override void OnAfterArrange()
+		{
+			base.OnAfterArrange();
+
+			if (_pendingChangeView is {} req)
+			{
+				var success = ChangeViewNative(req.horizontal, req.vertical, null, req.disableAnimation);
+				if (success || !IsArrangeDirty)
+				{
+					_pendingChangeView = default;
+				}
+			}
+		}
+
 		private bool ChangeViewNative(double? horizontalOffset, double? verticalOffset, float? zoomFactor, bool disableAnimation)
 		{
 			if (_scrollableContainer != null)
 			{
 				// iOS doesn't limit the offset to the scrollable bounds by itself
-				var newOffset = new CGPoint(horizontalOffset ?? HorizontalOffset, verticalOffset ?? VerticalOffset)
-					.Clamp(CGPoint.Empty, _scrollableContainer.UpperScrollLimit);
+				var limit = _scrollableContainer.UpperScrollLimit;
+				var desiredOffsets = new Foundation.Point(horizontalOffset ?? HorizontalOffset, verticalOffset ?? VerticalOffset);
+				var clampedOffsets = new Foundation.Point(MathEx.Clamp(desiredOffsets.X, 0, limit.X), MathEx.Clamp(desiredOffsets.Y, 0, limit.Y));
+
+				var success = desiredOffsets == clampedOffsets;
+				if (!success && IsArrangeDirty)
+				{
+					_pendingChangeView = (horizontalOffset, verticalOffset, disableAnimation);
+				}
 
-				_scrollableContainer.SetContentOffset(newOffset, !disableAnimation);
+				_scrollableContainer.SetContentOffset(desiredOffsets, !disableAnimation);
 
 				if(zoomFactor is { } zoom)
 				{
 					ChangeViewZoom(zoom, disableAnimation);
 				}
 
 				// Return true if successfully scrolled to asked offsets
-				return (horizontalOffset == null || horizontalOffset == newOffset.X) &&
-				       (verticalOffset == null || verticalOffset == newOffset.Y);
+				return success;
 			}
 
 			return false;