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

[martin rudalics]

>> 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.
>
> How would I check that?

If ...

> Looking more closely, it appears that I don't initially even have a
> scrollbar, even if I visit a file; it doesn't appear until I split the
> window.

... you never get an initial scroll bar, it would be indeed hard to
check that.  IIRC running Emacs from a console logs GTK errors in the
console output.  Can you try that?  Also could you send us screenshots
of your initial scroll-bar-less frame and the one where a scroll bar is
halfway visible somewhere in a window.

martin