bug#50424: 27.2; Tab bar button mouse face not clearing entirely

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@linkov.net>
> Cc: alan@idiocy.org,  luangruo@yahoo.com,  50424@debbugs.gnu.org
> Date: Mon, 13 Sep 2021 10:59:12 +0300
> 
> >> -#define DEFAULT_TAB_BAR_BUTTON_MARGIN 4
> >> +#define DEFAULT_TAB_BAR_BUTTON_MARGIN 1
> >>  #define DEFAULT_TAB_BAR_BUTTON_RELIEF 1
> >
> > Btw, I don't see much sense in keeping the
> > DEFAULT_TAB_BAR_BUTTON_MARGIN macro: it's used in one place only,
> > AFAICT, so we could simply have it there literally.
> 
> I thought about this too, but it will break the symmetry with
> DEFAULT_TOOL_BAR_BUTTON_MARGIN and DEFAULT_TOOL_BAR_BUTTON_RELIEF when
> DEFAULT_TAB_BAR_BUTTON_MARGIN will be used in more places.

The *_RELIEF variables are used in more than one place, so it makes
sense to have them.

As for the "symmetry" with the tool bar: it doesn't really exist,
since you do almost everything related to the tab-bar button display
in Lisp, not in C.  That "symmetry" is very misleading, so much so
that the other day it cost me 2 hours of frustrating debugging before
I realized why the image parameters I see in w32term.c are so
different from what Vtab_bar_button_margin says it should be.