Subject: bug#75993: Special mode-class for diff-mode

From: Juri Linkov <[email-masked]>
Date: Mon, 03 Feb 2025 09:56:14 +0200

>> Visiting a diff file and trying to copy lines with 'w' (diff-kill-ring-save)
>> does something unexpected since view-mode overrides 'w' with
>> 'View-scroll-page-backward-set-page-size' for non-nil 'view-read-only'.
>>
>> Here is the fix like for all modes that use single-letter keys:
> [...]
>> +(put 'diff-mode 'mode-class 'special)
>>  ;;;###autoload
>>  (define-derived-mode diff-mode fundamental-mode "Diff"
>>    "Major mode for viewing/editing context diffs.
>
> Hmm... this doesn't smell right:
>
> - I hate distinguishing between "mode-class = special" and "derives from
>   `special-mode`".
> - Whether we want to do that depends on `diff-mode-read-only` which is
>   buffer-local.

AFAICS, the special mode-class is handled only in `read-only-mode`:

   ((and buffer-read-only view-read-only
         (not view-mode)
         (not (eq (get major-mode 'mode-class) 'special)))
    (view-mode-enter))

This is exactly what is needed in diff-mode when it switches to read-only
it should not enable view-mode.