Subject: bug#32002: 24.4; Scroll bar start, end not correct

From: Glenn Morris <[email-masked]>
Date: Mon, 02 Jul 2018 14:16:54 -0400

>> I guess it's GTK-specific, then.  Something related to scaling,
>> perhaps?
[...]
> If so, perhaps you can advise me how to turn it off.  Usually
> the Emacs community is good enough to provide a way of
> disabling such features.

You can try building Emacs using --without-toolkit-scroll-bars.
(Despite the misleading --help text, this also affects GTK builds.)
Otherwise I imagine Emacs has no way to customize how GTK scroll bars work.