[platform-swt-dev] Drag images and background during DND on Windows

[Stephan Aßmus <superstippi@xxxxxx>]

Hi,

I have a custom widget based on Canvas that displays some items that the 
user can drag sort. When the user drags something, I am using an Image 
of the dragged item as the drag image:

final DragSource source = new DragSource(this, DND.DROP_MOVE);
source.setDragSourceEffect(new DragSourceEffect(this) {
	@Override
	public void dragStart(DragSourceEvent event) {
		event.image = // do some stuff to generate an image
	}
});

This worked fine until I've implemented animating the items in the 
Canvas to indicate to the user how items will be rearranged if a drop 
were to occur. That is, it still works perfectly on all platforms (GTK, 
Mac) except Windows. On Windows it looks like whenever I move the mouse, 
the platform takes a snapshot of the current Canvas contents, composes 
the drag image on top and places the (non transparent) result on screen. 
Obviously that mechanism has horrible results when the background is 
actually animated and changes while the mouse doesn't move. The area 
where the drag image appears still displays the contents at the time of 
the last mouse move, while the area around the drag image is animated 
and updated.

The solution that comes to mind is to implement a drag image myself by 
manually moving a transparent, custom shaped Shell along with the mouse. 
Before I go and do that, is my interpretation of the problem correct, 
and is there perhaps a simple solution that would fix the drag image 
rendering against an animated background on Windows?

Thanks for any help!

Best regards,
-Stephan