Dragging a region can only relocate it by its end point?

[earhax]

When I drag a region, it shows the pointer icon at the start of it. But as soon as I move the mouse past the end of the region, the pointer icon jumps to pointing at the end of the region, and then will follow the mouse past that point. When I release the mouse button, the end of the region will have moved to the position where the mouse pointer is. How can I get it to always move the *start* of the region to the mouse position?