bug#20619: Scrollbars

[Eli Zaretskii <eliz@gnu.org>]

> From: Eugen Dedu <eugen.dedu@univ-fcomte.fr>
> Date: Fri, 23 Dec 2016 21:30:17 +0100
> 
> Now, to advance emacs support for HIDPI I would like to fix the 
> scrollbar.  Do all people here agree that the scrollbar has a width 
> twice as normal?  The reason is that in src/gtkutil.c there is this code:
> 
> int
> xg_get_default_scrollbar_width (void)
> {
>    return scroll_bar_width_for_theme * xg_get_gdk_scale ();
> }
> 
> where xg_get_gdk_scale returns GDK_SCALE variable, i.e. 2 in general.
> 
> If I replace with:
>    return scroll_bar_width_for_theme;
> the scrollbar is shown correctly.
> 
> This change was made by 
> https://github.com/emacs-mirror/emacs/commit/c0055ff5b03c9121ab5bf752496b09416f0f0a7d. 
>   I think there was an error there, or perhaps in the mean time (since 
> May 2015) GTK has changed in a way so that scrollbars are taken into 
> account.

What is your version of GTK?  That commit points to a bug report
(bug#20432), so this change is not a mistake, it did fix a real
problem with scroll bars.  We could make it conditional on the GTK
version, though.  The bug report mentions a specific GTK version.

> Note that GDK_DPI_SCALE is only for font, AFAIU from 
> https://developer.gnome.org/gtk3/stable/gtk-x11.html.

The code you mention doesn't use GDK_DPI_SCALE.

> What do you think?  Would you commit such a modification?

I don't think we can simply revert the change in question, but maybe
we could use different code based on GTK version.

> I would like to look into other issues as well.

Thank you!  I see bugs 20432, 21469, and 18429 that might be relevant.