bug#46396: 27.1.90; Ediff's non-focused diff section, background too light in --reverse

[Matt Armstrong <matt@rfc20.org>]

bug-gnu-emacs_at_gnu.org@tangential.info writes:

> I created a file a.txt with foo\n\nbar\n\nbaz\n and a file b.txt with foo0\n\nbar\n\nbaz1\n.
>
> I open these files with "emacs -Q --reverse a.txt b.txt", and run M-x
> ediff-buffers RET for the two corresponding buffers.
>
> If I press n or p to traverse the diff sections, the diff section in focus
> is readable, but the unfocused sections have a white background, in other
> words, the contrast seems too low. The unfocused sections are fine without
> the --reverse option.
>
> Aside: Similar to --reverse, I have been using (invert-face 'default) for a
> simple dark theme in emacs. I can get a similar result in terminal emacs
> with "(setq frame-background-mode 'dark) (mapc 'frame-set-background-mode
> (frame-list))". These approaches to a simple dark theme came up in
> discussions on #emacs.
>
> I encountered the issue above, as I was using magit, with (invert-face
> 'default), and pressed `e` on a commit, wanting to view the diff in the
> side-by-side view.
>
> Thank you,
> -Brady
>
> Data from M-x report-emacs-bug RET below:
>
> In GNU Emacs 27.1.90 (build 1, x86_64-apple-darwin19.6.0, NS appkit-1894.60 Version 10.15.7 (Build 19H2))
> of 2021-01-15 built on clay.local
> Repository revision: 488204cdc64b6a130042ecc64d59c4538287b81d
> Repository branch: emacs-27
> Windowing system distributor 'Apple', version 10.3.1894
> System Description:  Mac OS X 10.15.7

I can confirm that the colors look bad on my system as well. The problem
does not seem confined to use of --reverse-video. They look bad if I set
the theme to a dark theme through "M-x customize-theme". For example,
the "deeper-blue" and "tango-dark" themes tickle the same problem. I
suspect any theme that sets the foreground color to a light color will
exhibit this.

I notice that one of the bad looking diff faces is `ediff-even-diff-A',
which, on any system with >88 colors, the face fails to specify a
foreground color. Most ediff-* faces are like this.

I wonder if faces should always set foreground/background colors in
pairs?

Anyway, when I expressly set the foreground colors for these faces to
black, they look fine in dark themes.