It might be easier to use 2 picture controls. One for displaying the window glass. One for display in the circle.
You can one on top of another, and make the background transparent. To display circle without background, you can use mask.
Moving the circle inside the picture control is easy. You don't need the drag and drop event. All you need is mouse down, mouse move, mouse up event.
When mouse down, register mouse move, and mouse up event. In mouse move event, reset the picture origin.
When mouse up, unregister mouse move, mouse up event.
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