Re: Duplicated outline-cycle binding, and problems with the new one

[Juri Linkov]

>> But diff-mode overrides this binding with diff-mode-shared-map
>> where the TAB key is bound to diff-hunk-next, since
>> minor-mode-overriding-map-alist takes priority over
>> minor-mode-map-alist when diff-mode does this:
>
> I see.  I knew using `minor-mode-overriding-map-alist` in `diff-mode.el`
> this way was going to bite us sooner or later.
>
> Maybe we should use a hook on `read-only-mode` to set/unset
> a `diff-mode-read-only` variable so we can add the keymap
> (conditionalized on this new `diff-mode-read-only`) to
> `minor-mode-map-alist` instead of `minor-mode-overriding-map-alist`.

So this is because `minor-mode-map-alist` is not buffer-local.
Then this requires changing `(setq buffer-read-only t)` to
`(read-only-mode 1)` in diff-related places.  Since `read-only-mode`
always activates `view-mode` when `view-read-only` is t,
it needs let-binding: (let ((view-read-only nil)) (read-only-mode 1)).
This will keep the current behavior.  Then special-handling of
`view-mode` in `diff-mode` is not needed because `view-mode`
is higher than `diff-mode-read-only` in `minor-mode-map-alist`,
where `diff-mode-read-only` is at the end to not take precedence
over `outline-minor-mode`.

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index ca8df5d380..63b9549b54 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -1479,6 +1479,12 @@ diff--font-lock-cleanup
 (defvar whitespace-style)
 (defvar whitespace-trailing-regexp)
 
+(defvar-local diff-mode-read-only nil)
+
+(or (assq 'diff-mode-read-only minor-mode-map-alist)
+    (nconc minor-mode-map-alist
+           (list (cons 'diff-mode-read-only diff-mode-shared-map))))
+
 ;;;###autoload
 (define-derived-mode diff-mode fundamental-mode "Diff"
   "Major mode for viewing/editing context diffs.
@@ -1516,23 +1522,19 @@ diff-mode
 
   (diff-setup-whitespace)
 
+  (add-hook 'read-only-mode-hook
+            (lambda ()
+              (setq diff-mode-read-only buffer-read-only))
+            nil t)
+
   (if diff-default-read-only
-      (setq buffer-read-only t))
+      (let ((view-read-only nil)) (read-only-mode 1)))
   ;; setup change hooks
   (if (not diff-update-on-the-fly)
       (add-hook 'write-contents-functions #'diff-write-contents-hooks nil t)
     (make-local-variable 'diff-unhandled-changes)
     (add-hook 'after-change-functions #'diff-after-change-function nil t)
     (add-hook 'post-command-hook #'diff-post-command-hook nil t))
-  ;; Neat trick from Dave Love to add more bindings in read-only mode:
-  (let ((ro-bind (cons 'buffer-read-only diff-mode-shared-map)))
-    (add-to-list 'minor-mode-overriding-map-alist ro-bind)
-    ;; Turn off this little trick in case the buffer is put in view-mode.
-    (add-hook 'view-mode-hook
-             (lambda ()
-               (setq minor-mode-overriding-map-alist
-                     (delq ro-bind minor-mode-overriding-map-alist)))
-             nil t))
   ;; add-log support
   (setq-local add-log-current-defun-function #'diff-current-defun)
   (setq-local add-log-buffer-file-name-function
diff --git a/lisp/vc/diff.el b/lisp/vc/diff.el
index 4abcf6c15a..425155be40 100644
--- a/lisp/vc/diff.el
+++ b/lisp/vc/diff.el
@@ -182,7 +182,7 @@ diff-no-select
                     " "))
         (thisdir default-directory))
     (with-current-buffer buf
-      (setq buffer-read-only t)
+      (let ((view-read-only nil)) (read-only-mode 1))
       (buffer-disable-undo (current-buffer))
       (let ((inhibit-read-only t))
        (erase-buffer))
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 6041c79efc..600c4561ac 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -1817,7 +1826,7 @@ vc-diff-internal
     ;; Make the *vc-diff* buffer read only, the diff-mode key
     ;; bindings are nicer for read only buffers. pcl-cvs does the
     ;; same thing.
-    (setq buffer-read-only t)
+    (let ((view-read-only nil)) (read-only-mode 1))
     (if (and (zerop (buffer-size))
              (not (get-buffer-process (current-buffer))))
         ;; Treat this case specially so as not to pop the buffer.