X axis panel dragging traces back to Blender versions before 2.5, where panels could be aligned horizontally. But for many years now panels have been vertically aligned. Considering this, keeping the X axis dragging around is a bit odd. It makes interaction confusing, or at least more complicated. It also looks bad, since any part of the panel outside the region is cropped. Differential Revision: https://developer.blender.org/D9549