bug#42300: Visual bug in diff-mode

bug#42300: Visual bug in diff-mode

[Gregory Heytings via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Diff-mode was updated in Emacs 27, and it is now possible to use the 
fringe to indicate whether the lines were added, deleted, or are common. 
The face is omitted in the display property in the case common lines, 
which leads to the bug described below.

Steps to reproduce:

1. start emacs -Q
2. (setq diff-font-lock-prettify t)
3. to make the bug clearly visible: (set-face-attribute 'fringe nil :background "red" :foreground "yellow")
4. open a diff file (with "common" lines, that is, lines starting with a space)
5. observe that the fringe in front of the common lines is red
6. press C-x C-+, the fringe in front of the common lines is now white
7. press -, the fringe in front of the common lines is now red again

Fix:

--- diff-mode.el.orig	2020-06-29 17:39:26.000000000 +0000
+++ diff-mode.el	2020-07-09 18:44:08.000000000 +0000
@@ -2518,7 +2518,7 @@
                                 '((?+ . (left-fringe diff-fringe-add diff-indicator-added))
                                   (?- . (left-fringe diff-fringe-del diff-indicator-removed))
                                   (?! . (left-fringe diff-fringe-rep diff-indicator-changed))
-                                 (?\s . (left-fringe diff-fringe-nul))))))
+                                 (?\s . (left-fringe diff-fringe-nul fringe))))))
            (put-text-property (match-beginning 0) (match-end 0) 'display spec))))
      ;; Mimicks the output of Magit's diff.
      ;; FIXME: This has only been tested with Git's diff output.

bug#42300: Visual bug in diff-mode

[Stefan Kangas]

tags 42300 + confirmed
found 42300 28.0.50
thanks

Gregory Heytings <ghe@sdf.org> writes:

> Diff-mode was updated in Emacs 27, and it is now possible to use the fringe to
> indicate whether the lines were added, deleted, or are common. The face is
> omitted in the display property in the case common lines, which leads to the bug
> described below.
>
> Steps to reproduce:
>
> 1. start emacs -Q
> 2. (setq diff-font-lock-prettify t)
> 3. to make the bug clearly visible: (set-face-attribute 'fringe nil :background "red" :foreground "yellow")
> 4. open a diff file (with "common" lines, that is, lines starting with a space)
> 5. observe that the fringe in front of the common lines is red
> 6. press C-x C-+, the fringe in front of the common lines is now white
> 7. press -, the fringe in front of the common lines is now red again

I can reproduce this on current master, and verify that the below change
fixes it.

I'm not that familiar with diff-mode.ela nor font-locking so it's better
that someone else takes a look and installs the fix.

> --- diff-mode.el.orig	2020-06-29 17:39:26.000000000 +0000
> +++ diff-mode.el	2020-07-09 18:44:08.000000000 +0000
> @@ -2518,7 +2518,7 @@
>                                 '((?+ . (left-fringe diff-fringe-add diff-indicator-added))
>                                   (?- . (left-fringe diff-fringe-del diff-indicator-removed))
>                                   (?! . (left-fringe diff-fringe-rep diff-indicator-changed))
> -                                 (?\s . (left-fringe diff-fringe-nul))))))
> +                                 (?\s . (left-fringe diff-fringe-nul fringe))))))
>            (put-text-property (match-beginning 0) (match-end 0) 'display spec))))
>      ;; Mimicks the output of Magit's diff.
>      ;; FIXME: This has only been tested with Git's diff output.

Best regards,
Stefan Kangas

bug#42300: Visual bug in diff-mode

[Lars Ingebrigtsen]

Stefan Kangas <stefan@marxist.se> writes:

> I can reproduce this on current master, and verify that the below change
> fixes it.
>
> I'm not that familiar with diff-mode.ela nor font-locking so it's better
> that someone else takes a look and installs the fix.

I'm not very familiar with that, either, but it seems "obviously
correct", if I understand that code correctly.  So I went ahead and
pushed it to Emacs 28.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no