fix(dragdrop): Fix support of transparent control in DragUI

src/Uno.Foundation/Point.cs

@@ -37,6 +37,8 @@ internal string ToDebugString()
 
 		internal Point WithY(double y) => new Point(X, y);
 
+		internal Point GetOpposite() => new Point(-X, -Y);
+
 		public static bool operator ==(Point left, Point right)
 		{
 			return left.Equals(right);

src/Uno.UI/UI/Xaml/DragDrop/DragUI.cs

@@ -10,12 +10,10 @@ public partial class DragUI
 	{
 		internal ImageSource? Content { get; set; }
 
-		internal Point? Anchor { get; private set; }
+		internal Point? Anchor { get; set; }
 
 		public void SetContentFromBitmapImage(BitmapImage bitmapImage)
-		{
-			Content = bitmapImage;
-		}
+			=> SetContentFromBitmapImage(bitmapImage, default);
 
 		public void SetContentFromBitmapImage(BitmapImage bitmapImage, Point anchorPoint)
 		{

src/Uno.UI/UI/Xaml/DragDrop/DragView.cs

@@ -69,6 +69,17 @@ public ImageSource? Content
 		}
 		#endregion
 
+		#region ContentAnchor
+		public static readonly DependencyProperty ContentAnchorProperty = DependencyProperty.Register(
+			"ContentAnchor", typeof(Point), typeof(DragView), new FrameworkPropertyMetadata(default(Point)));
+
+		public Point ContentAnchor
+		{
+			get => (Point)GetValue(ContentAnchorProperty);
+			private set => SetValue(ContentAnchorProperty, value);
+		}
+		#endregion
+
 		#region ContentVisibility
 		public static readonly DependencyProperty ContentVisibilityProperty = DependencyProperty.Register(
 			"ContentVisibility", typeof(Visibility), typeof(DragView), new FrameworkPropertyMetadata(default(Visibility)));
@@ -110,8 +121,8 @@ public void SetLocation(Point location)
 			// TODO: Make sure to not move the element out of the bounds of the window
 			_location = location;
 
-			_transform.X = location.X - (ActualWidth / 2);
-			_transform.Y = location.Y - 40; // The caption is above the pointer
+			_transform.X = location.X;
+			_transform.Y = location.Y;
 		}
 
 		public void Update(DataPackageOperation acceptedOperation, CoreDragUIOverride viewOverride)
@@ -132,7 +143,16 @@ public void Update(DataPackageOperation acceptedOperation, CoreDragUIOverride vi
 			GlyphVisibility = ToVisibility(viewOverride.IsGlyphVisible);
 			Caption = caption!;
 			CaptionVisibility = ToVisibility(viewOverride.IsCaptionVisible && !string.IsNullOrWhiteSpace(caption));
-			Content = viewOverride.Content as ImageSource ?? _ui?.Content;
+			if (viewOverride.Content is ImageSource overridenContent)
+			{
+				Content = overridenContent;
+				ContentAnchor = viewOverride.ContentAnchor;
+			}
+			else
+			{
+				Content = _ui?.Content;
+				ContentAnchor = _ui?.Anchor ?? default;
+			}
 			ContentVisibility = ToVisibility(viewOverride.IsContentVisible);
 			TooltipVisibility = ToVisibility(viewOverride.IsGlyphVisible || viewOverride.IsCaptionVisible);
 			Visibility = Visibility.Visible;

src/Uno.UI/UI/Xaml/DragDrop/DragView.xaml

@@ -14,7 +14,7 @@
 	xmlns:uBehaviors="using:Uno.UI.Behaviors"
 	mc:Ignorable="d ios android wasm netstdref macos skia">
 
-	<!-- Default style for Windows.UI.Xaml.Controls.ScrollViewer -->
+	<!-- Default style for internal control Windows.UI.Xaml.DragView -->
 	<Style TargetType="DragView">
 		<Setter Property="IsTabStop" Value="False" />
 		<Setter Property="IsHitTestVisible" Value="False" />
@@ -26,14 +26,24 @@
 			<Setter.Value>
 				<ControlTemplate TargetType="DragView">
 					<Grid HorizontalAlignment="Left" VerticalAlignment="Top">
-						<Image Visibility="{TemplateBinding ContentVisibility}" Source="{TemplateBinding Content}" Opacity=".8" />
+						<Image Visibility="{TemplateBinding ContentVisibility}" Source="{TemplateBinding Content}" Opacity=".8">
+							<Image.RenderTransform>
+								<TranslateTransform X="{TemplateBinding ContentAnchor.X}" Y="{TemplateBinding ContentAnchor.Y}" />
+							</Image.RenderTransform>
+						</Image>
 						<StackPanel
+							HorizontalAlignment="Left"
+							VerticalAlignment="Top"
 							Orientation="Horizontal"
 							BorderThickness="1"
 							BorderBrush="{StaticResource SystemControlForegroundChromeHighBrush}"
 							Background="{StaticResource SystemControlBackgroundChromeMediumLowBrush}"
 							Padding="2,5"
 							Visibility="{TemplateBinding TooltipVisibility}">
+							<StackPanel.RenderTransform>
+								<!-- The caption is above the pointer -->
+								<TranslateTransform Y="-40" />
+							</StackPanel.RenderTransform>
 							<TextBlock Visibility="{TemplateBinding GlyphVisiblity}" Text="{TemplateBinding Glyph}" Margin="3,0" />
 							<TextBlock Visibility="{TemplateBinding CaptionVisiblity}" Text="{TemplateBinding Caption}" Margin="3,0" />
 						</StackPanel>

src/Uno.UI/UI/Xaml/Media/Imaging/RenderTargetBitmap.Android.cs

@@ -1,6 +1,9 @@
-using System;
+#nullable enable
+
+using System;
 using System.IO;
 using System.Linq;
+using System.Numerics;
 using Android.Graphics;
 using Uno.UI;
 using Windows.Foundation;
@@ -13,29 +16,31 @@ partial class RenderTargetBitmap
 		private protected override bool IsSourceReady => _buffer != null;
 
 		/// <inheritdoc />
-		private protected override bool TryOpenSourceSync(int? targetWidth, int? targetHeight, out Bitmap image)
+		private protected override bool TryOpenSourceSync(int? targetWidth, int? targetHeight, out Bitmap? image)
 		{
 			image = BitmapFactory.DecodeByteArray(_buffer, 0, _buffer.Length);
 			return image != null;
 		}
 
 		private static byte[] RenderAsPng(UIElement element, Size? scaledSize = null)
 		{
-			var width = (int)ViewHelper.LogicalToPhysicalPixels(element.ActualSize.X);
-			var height = (int)ViewHelper.LogicalToPhysicalPixels(element.ActualSize.Y);
-			var bitmap = Bitmap.CreateBitmap(width, height, Bitmap.Config.Argb8888);
+			var logical = element.ActualSize.ToSize();
+			var physical = logical.LogicalToPhysicalPixels();
+			var bitmap = Bitmap.CreateBitmap((int)physical.Width, (int)physical.Height, Bitmap.Config.Argb8888!)
+				?? throw new InvalidOperationException("Failed to create target native bitmap.");
 			var canvas = new Canvas(bitmap);
 
 			// Make sure the element has been Layouted 
-			element.Layout(0, 0, width, height);
+			element.Layout(0, 0, (int)physical.Width, (int)physical.Height);
 
 			// Render on the canvas
-			canvas.DrawColor(Colors.White);
+			canvas.DrawColor(Colors.Transparent);
 			element.Draw(canvas);
 
-			if (scaledSize.HasValue)
+			if (scaledSize is {} targetSize)
 			{
-				canvas.Scale((float)(scaledSize.Value.Width / (float)width), (float)(scaledSize.Value.Height / (float)height));
+				bitmap = Bitmap.CreateScaledBitmap(bitmap, (int)targetSize.Width, (int)targetSize.Height, false)
+					?? throw new InvalidOperationException("Failed to scaled native bitmap to the requested size.");
 			}
 
 			using var stream = new MemoryStream();

src/Uno.UI/UI/Xaml/Media/Imaging/RenderTargetBitmap.iOS.cs

@@ -25,9 +25,9 @@ private byte[] RenderAsPng(UIElement element, Size? scaledSize = null)
 			UIImage img;
 			try
 			{
-				UIGraphics.BeginImageContextWithOptions(new Size(element.ActualSize.X, element.ActualSize.Y), true, 1f);
+				UIGraphics.BeginImageContextWithOptions(new Size(element.ActualSize.X, element.ActualSize.Y), false, 1f);
 				var ctx = UIGraphics.GetCurrentContext();
-				ctx.SetFillColor(Colors.White);
+				ctx.SetFillColor(Colors.Transparent); // This is only for pixels not used, but the bitmap as the same size of the element. We keep it only for safety!
 				element.Layer.RenderInContext(ctx);
 				img = UIGraphics.GetImageFromCurrentImageContext();
 			}

src/Uno.UI/UI/Xaml/UIElement.Pointers.cs

@@ -259,7 +259,7 @@ internal HitTestability GetHitTestVisibility()
 #endif
 		}
 
-				#region GestureRecognizer wire-up
+		#region GestureRecognizer wire-up
 
 				#region Event to RoutedEvent handler adapters
 		// Note: For the manipulation and gesture event args, the original source has to be the element that raise the event
@@ -391,8 +391,8 @@ private async void HapticFeedbackWhenReadyToDrag(GestureRecognizer sender, Gestu
 			}
 		}
 				#endregion
-
-				#region Manipulations (recognizer settings / custom bubbling)
+		
+		#region Manipulations (recognizer settings / custom bubbling)
 		partial void AddManipulationHandler(RoutedEvent routedEvent, int handlersCount, object handler, bool handledEventsToo)
 		{
 			if (handlersCount == 1)
@@ -449,7 +449,7 @@ partial void PrepareManagedManipulationEventBubbling(RoutedEvent routedEvent, re
 		}
 				#endregion
 
-				#region Gestures (recognizer settings / custom bubbling / early completion)
+		#region Gestures (recognizer settings / custom bubbling / early completion)
 		private bool _isGestureCompleted;
 
 		partial void AddGestureHandler(RoutedEvent routedEvent, int handlersCount, object handler, bool handledEventsToo)
@@ -522,7 +522,7 @@ private protected void CompleteGesture()
 		}
 				#endregion
 
-				#region Drag And Drop (recognizer settings / custom bubbling / drag starting event)
+		#region Drag And Drop (recognizer settings / custom bubbling / drag starting event)
 		private void UpdateDragAndDrop(bool isEnabled)
 		{
 			// Note: The drag and drop recognizer setting is only driven by the CanDrag,
@@ -640,6 +640,10 @@ private async Task<DataPackageOperation> StartDragAsyncCore(PointerPoint pointer
 			PrepareShare(routedArgs.Data); // Gives opportunity to the control to fulfill the data
 			SafeRaiseEvent(DragStartingEvent, routedArgs); // The event won't bubble, cf. PrepareManagedDragAndDropEventBubbling
 
+			// We capture the original position of the pointer before going async,
+			// so we have the closet location of the "down" possible.
+			var ptPosition = ptArgs.GetCurrentPoint(this).Position;
+
 			if (routedArgs.Deferral is { } deferral)
 			{
 				await deferral.Completed(ct);
@@ -666,9 +670,14 @@ private async Task<DataPackageOperation> StartDragAsyncCore(PointerPoint pointer
 
 			if (RenderTargetBitmap.IsImplemented && routedArgs.DragUI.Content is null)
 			{
+				// Note: Bitmap rendered by the RenderTargetBitmap is in physical pixels,
+				//		 so we provide the ActualSize to request the image to be scaled back in logical pixels. 
+
 				var target = new RenderTargetBitmap();
-				await target.RenderAsync(this);
+				await target.RenderAsync(this, (int)ActualSize.X, (int)ActualSize.Y);
+
 				routedArgs.DragUI.Content = target;
+				routedArgs.DragUI.Anchor = ptPosition.GetOpposite();
 			}
 
 			var asyncResult = new TaskCompletionSource<DataPackageOperation>();
@@ -756,7 +765,7 @@ partial void PrepareManagedPointerEventBubbling(RoutedEvent routedEvent, ref Rou
 			}
 		}
 
-				#region Partial API to raise pointer events and gesture recognition (OnNative***)
+		#region Partial API to raise pointer events and gesture recognition (OnNative***)
 		private bool OnNativePointerEnter(PointerRoutedEventArgs args, BubblingContext ctx = default) => OnPointerEnter(args);
 
 		private bool OnPointerEnter(PointerRoutedEventArgs args, BubblingContext ctx = default)