bug#5571: 23.1.92; scroll bar disappears when dragging an edge between mode lines horizontally

[Jan Djärv]

YAMAMOTO Mitsuharu skrev:
> Steps to reproduce:
>
>   1. emacs -Q
>   2. C-x 3
>   (2.5. Optionally, C-x o followed by C-x 2)
>   3. Drag the edge between the mode lines at the bottom center in
>      order to move the scroll bar horizontally.
>
> Result:
>
>   The scroll bar at the center disappears until you stop dragging.
>   This does not happen when GDK_NATIVE_WINDOWS=1.

I checked in a fix.  The scroll bar in the middle flickers a lot during drag, 
but at least it doesn't dissapear.

        Jan D.