[REG 5.3->5.4] Windows 7: Scrollbar gripper is not hidden when scroll handle is too small for gripper

On Windows, if you resize a window containing scroll bars to be small enough that the scroll bar's handle gets very small, the gripper on the handle should disappear. However, the gripper is instead drawn at (0, 0) of the scroll bar instead of being hidden.

The attached screenshots show the qtbase/examples/widgets/widgets/sliders/sliders.pro example experiment. To reproduce the bug, resize the window to the smallest possible size and notice how the horizontal scroll bar has the gripper drawn at (0, 0).

I believe that this commit was introduced in 176bd1edfadcb68b914a27de75502edeb1081253, which references QTBUG-38993 and QTBUG-38858.