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

[Juri Linkov]

> The origin of the faces without a foreground setting is commit
> 382ceb2cdbedc06b06d9fb424d83f531339a3311 by Juri Linkov <juri@jurta.org>
> while working on https://debbugs.gnu.org/cgi/bugreport.cgi?bug=10181.
>
> I've cc'd him here. Juri, can you comment?

Thanks for pointing me at this issue since I had not realized
this is related to something affected by my previous changes.

> I'm especially interested to hear your thoughts on why the change made
> things nicer (what you said in
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=10181#84).

The core point of my 2014-year message was that with the specified foreground,
Ediff removes font-lock highlighting on non-current differences.
The need for keeping the font-lock foreground colors is evident
when looking at these font-lock colors in buffers where the effect
of font-lock is noticeable, such as changes in the source code files.

But I completely agree that the current contrast is too low on a dark theme.

Fortunately, now we have a feature that will help to fix this problem:
the face attribute :distant-foreground specifies the alternative
foreground color that is used only when the background color is too
close to the foreground color, thus making the text readable in this case.

> Or, perhaps the ediff faces should vary the background based on
> (background light) and (background dark)?

The background colors could be adjusted as well for dark backgrounds.

After changing background colors to darker colors and adding
:distant-foreground I tried different default foreground colors with

  M-x set-foreground-color RET dark grey RET

and it seems everything looks readable.

Do you see more problems with the following patch?

diff --git a/lisp/vc/ediff-init.el b/lisp/vc/ediff-init.el
index 6e658163b9..3f33e6aae2 100644
--- a/lisp/vc/ediff-init.el
+++ b/lisp/vc/ediff-init.el
@@ -980,8 +980,10 @@ stipple-pixmap
 (defface ediff-even-diff-A
   `((((type pc))
      (:foreground "green3" :background "light grey" :extend t))
-    (((class color) (min-colors 88))
-     (:background "light grey" :extend t))
+    (((class color) (min-colors 88) (background light))
+     (:distant-foreground "Black" :background "light grey" :extend t))
+    (((class color) (min-colors 88) (background dark))
+     (:distant-foreground "White" :background "dark grey" :extend t))
     (((class color) (min-colors 16))
      (:foreground "Black" :background "light grey" :extend t))
     (((class color))
@@ -999,8 +1001,10 @@ ediff-even-diff-face-A
 this variable represents.")
 
 (defface ediff-even-diff-B
-  `((((class color) (min-colors 88))
-     (:background "Grey" :extend t))
+  `((((class color) (min-colors 88) (background light))
+     (:distant-foreground "Black" :background "Grey" :extend t))
+    (((class color) (min-colors 88) (background dark))
+     (:distant-foreground "White" :background "dim grey" :extend t))
     (((class color) (min-colors 16))
      (:foreground "White" :background "Grey" :extend t))
     (((class color))
@@ -1019,8 +1023,10 @@ ediff-even-diff-face-B
 (defface ediff-even-diff-C
   `((((type pc))
      (:foreground "yellow3" :background "light grey" :extend t))
-    (((class color) (min-colors 88))
-     (:background "light grey" :extend t))
+    (((class color) (min-colors 88) (background light))
+     (:distant-foreground "Black" :background "light grey" :extend t))
+    (((class color) (min-colors 88) (background dark))
+     (:distant-foreground "White" :background "dark grey" :extend t))
     (((class color) (min-colors 16))
      (:foreground "Black" :background "light grey" :extend t))
     (((class color))
@@ -1040,8 +1046,10 @@ ediff-even-diff-face-C
 (defface ediff-even-diff-Ancestor
   `((((type pc))
      (:foreground "cyan3" :background "light grey" :extend t))
-    (((class color) (min-colors 88))
-     (:background "Grey" :extend t))
+    (((class color) (min-colors 88) (background light))
+     (:distant-foreground "Black" :background "Grey" :extend t))
+    (((class color) (min-colors 88) (background dark))
+     (:distant-foreground "White" :background "dim grey" :extend t))
     (((class color) (min-colors 16))
      (:foreground "White" :background "Grey" :extend t))
     (((class color))
@@ -1068,8 +1076,10 @@ ediff-even-diff-face-alist
 (defface ediff-odd-diff-A
   '((((type pc))
      (:foreground "green3" :background "gray40" :extend t))
-    (((class color) (min-colors 88))
-     (:background "Grey" :extend t))
+    (((class color) (min-colors 88) (background light))
+     (:distant-foreground "Black" :background "Grey" :extend t))
+    (((class color) (min-colors 88) (background dark))
+     (:distant-foreground "White" :background "dim grey" :extend t))
     (((class color) (min-colors 16))
      (:foreground "White" :background "Grey" :extend t))
     (((class color))
@@ -1088,8 +1098,10 @@ ediff-odd-diff-face-A
 (defface ediff-odd-diff-B
   '((((type pc))
      (:foreground "White" :background "gray40" :extend t))
-    (((class color) (min-colors 88))
-     (:background "light grey" :extend t))
+    (((class color) (min-colors 88) (background light))
+     (:distant-foreground "Black" :background "light grey" :extend t))
+    (((class color) (min-colors 88) (background dark))
+     (:distant-foreground "White" :background "dark grey" :extend t))
     (((class color) (min-colors 16))
      (:foreground "Black" :background "light grey" :extend t))
     (((class color))
@@ -1108,8 +1120,10 @@ ediff-odd-diff-face-B
 (defface ediff-odd-diff-C
   '((((type pc))
      (:foreground "yellow3" :background "gray40" :extend t))
-    (((class color) (min-colors 88))
-     (:background "Grey" :extend t))
+    (((class color) (min-colors 88) (background light))
+     (:distant-foreground "Black" :background "Grey" :extend t))
+    (((class color) (min-colors 88) (background dark))
+     (:distant-foreground "White" :background "dim grey" :extend t))
     (((class color) (min-colors 16))
      (:foreground "White" :background "Grey" :extend t))
     (((class color))