Use case

I have a draggable window for which the title bar acts as a handle. So it is only possible to drag the window by clicking in the title bar at the top of the window. I've set the containment of the window so it cannot be dragged outside the view port.

I have two droppable elements at the bottom of the view port, one on top of the other. The droppable elements are not as tall as the draggable. There is no way to drag the window onto the lowermost droppable.

I tried setting the tolerance of the droppable elements to pointer to resolve this problem. Even though I could move the pointer over the bottom droppable element I could not drop the window on the bottom element.

The problem occurs because the code in droppable assumes that the mouse pointer is always in the same position relative to the draggable that it was in when the user clicked the mouse to start dragging, or that it is always at the cursorAt position. This invariant does not hold when containment has been specified for the draggable and the draggable is close to the boundaries of the "container".

How to reproduce

I've attached a patch that adds a unit test to reproduce the problem.

How to fix

I've attached a patch that fixes the problem.