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

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

>> Do we have something in defface that we can "recommend" to conditionally
>> specify this attribute when version >= 27 only (maybe a syntax sugar)?
>
> (if (>= emacs-major-version 27)
>     (defface foo...) ; for Emacs 27 and later
>   (defface foo...)   ; for Emacs 26 and older

I believe it is more complex. There might be situations foo shall not
extend to EOL, and other situations it should.

The latter case is when foo is used in a "rectangular" context as
explained. You need to introduce a second face foo-extend, and you must
replace all uses of foo by foo-extend where needed.

Best regards, Michael.