bug#34569: 26.1.90; Zero wide scroll bars

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 23 Feb 2019 15:01:54 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: 34569@debbugs.gnu.org
> 
>  > If turning off scroll-bar-mode does work, then how about disabling the
>  > direct setting of 'scroll-bar-width', either silently or noisily?
> 
> Setting 'scroll-bar-width' is fragile.  With GTK builds you can set
> it, Emacs will respect it and GTK will either overdraw or leave a gap
> because you can't change the GTK scroll bar from within Emacs.  Note
> the dual use of this parameter: 'scroll-bar-width' is (1) passed on to
> the toolkit to draw scroll bars of the desired width and (2) used by
> Emacs to clear various areas of the window and calculate the width of
> the text area.
> 
> But yes: One way to fix the aborts should be to disallow setting the
> 'scroll-bar-width' frame parameter to zero.

Then I think we should do that.

> This hints at another way of fixing the aborts: Handle zero wide
> scroll bars just as if scroll bars were disabled/turned off.

Yes.

>  > If we disallow setting this parameter directly, or at least setting it
>  > to zero, this problem will also go away, right?
> 
> Why would we disallow setting this parameter directly?  On Windows it
> works perfectly.  On Lucid and Motif all widths but zero work well.

I said "or at least setting it to zero".