bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[Dmitry Gutov]

On 23.10.2019 18:39, Eli Zaretskii wrote:

> > > I don't know, as I don't have a clear idea what faces there are
> > > affected and why.  I hoped someone, preferably the Magit developers,
> > > would describe that in enough detail to understand the situation.
> >
> > The list of faces has been posted here already:
> >
> > https://debbugs.gnu.org/cgi/bugreport.cgi?bug=37774#233
>
> AFAIU, that's a list of faces one particular user decided to customize
> to have them extended.  It's a far cry from the list of faces that
> actually need to be extended, lest some important functionality will
> suffer.  IOW, we need some rationale for each face, so that we could
> consider that and decide whether or not to extend each one by default.

Magit's maintainer will decide for each face, sure.

But I don't really see much a difference between having 2 and 20 faces 
that will need to be updated, if it's within one package.

Even if it's just 2, do we have a recommended way to write their 
definitions in third-party packages in a way that's compatible with 
Emacs 26?

> If too many faces in unbundled packages indeed need to change in that
> way, we should consider additional measures.  That's why we need good
> reasons for extending each face, not just "because they were before"
> or because people were used to see them extended.

Those are not the worst reasons, though.