bug#53636: 29.0.50; face-remapping broken on master

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 53636@debbugs.gnu.org,  tsdh@gnu.org
> Date: Sun, 30 Jan 2022 19:31:54 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > If the problem is with face remapping, can we please have a reproducer
> > that is simpler to debug?  Like one that doesn't need any timers and
> > flashing text?
> 
> I assumed that all that async stuff was part of the reproducer, but
> indeed it's not.
> 
> Just evaling this is sufficient:
> 
>  (face-remap-add-relative 'mode-line 'error)
> 
> And this is because mode-line is (in Emacs 29) the parent of two faces
> (but not used directly on the mode line), so you apparently have to do
> this instead:
> 
>  (face-remap-add-relative 'mode-line-active 'error)
> 
> I'm not sure whether that's a bug or not?  (That remapping a parent face
> has no effect.)

But it does have effect.  Try "C-x 5 b *scratch* RET" after evaluating
the first face-remap-add-relative, and you will see that it does have
effect.  So it's something more subtle...

And yes, I think the extra indirection we now have on master exposed a
problem we never saw before.