bug#21469: 25.0.50; right scrollbar wrongly placed when resizing split window

[martin rudalics]

> I'm starting to think this is either something in my environment or a
> GTK issue.

In xg_set_toolkit_scroll_bar_thumb in gtkutil.c could you check whether
_we_ already select the wrong bar->window?  Provided your earlier
assumption that "the right scrollbar controls the window to the left,
not the window to the right" holds, obviously.

martin