bug#57813: Icon images are non-functional

[Juri Linkov]

> Next I could try to add an option to display these outline images in
> margins.

Here is a preliminary patch, but using buffer-local left-margin-width
exhibits strange behavior: margins don't always appear immediately
after switching the buffer.

diff --git a/lisp/outline.el b/lisp/outline.el
index 25ef1616b9..3b8e68bfe1 100644
--- a/lisp/outline.el
+++ b/lisp/outline.el
@@ -292,6 +292,16 @@ outline-minor-mode-use-buttons
   :safe #'booleanp
   :version "29.1")
 
+(defcustom outline-minor-mode-use-margins '(derived-mode . special-mode)
+  "Whether to display clickable buttons on the margins.
+The value should be a `buffer-match-p' condition.
+
+These buttons can be used to hide and show the body under the heading.
+Note that this feature is meant to be used in editing buffers."
+  :type 'buffer-predicate
+  :safe #'booleanp
+  :version "29.1")
+
 (define-icon outline-open nil
   '((image "outline-open.svg" "outline-open.pbm"
            :height 15 :ascent center)
@@ -437,8 +447,10 @@ outline-minor-mode-highlight-buffer
                     (and (eq outline-minor-mode-highlight t)
                          (not (get-text-property (match-beginning 0) 'face))))
             (overlay-put overlay 'face (outline-font-lock-face)))
-          (when (outline--use-buttons-p)
-            (outline--insert-open-button)))
+          (cond ((outline--use-margins-p)
+                 (outline--insert-open-button t))
+                ((outline--use-buttons-p)
+                 (outline--insert-open-button))))
         (goto-char (match-end 0))))))
 
 ;;;###autoload
@@ -453,6 +465,9 @@ outline-minor-mode
             (key-description outline-minor-mode-prefix) 
outline-mode-prefix-map)
   (if outline-minor-mode
       (progn
+        (when (outline--use-margins-p)
+          (setq-local left-margin-width (1+ left-margin-width))
+          (setq-local fringes-outside-margins t))
         (when outline-minor-mode-highlight
           (if (and global-font-lock-mode (font-lock-specified-p major-mode))
               (progn
@@ -473,6 +488,9 @@ outline-minor-mode
           (font-lock-remove-keywords nil outline-font-lock-keywords))
       (remove-overlays nil nil 'outline-overlay t)
       (font-lock-flush))
+    (when (outline--use-margins-p)
+      (setq-local left-margin-width (1- left-margin-width))
+      (setq-local fringes-outside-margins nil))
     (setq line-move-ignore-invisible nil)
     ;; Cause use of ellipses for invisible text.
     (remove-from-invisibility-spec '(outline . t))
@@ -483,6 +501,10 @@ outline--use-buttons-p
   (and outline-minor-mode
        (buffer-match-p outline-minor-mode-use-buttons (current-buffer))))
 
+(defun outline--use-margins-p ()
+  (and outline-minor-mode
+       (buffer-match-p outline-minor-mode-use-margins (current-buffer))))
+
 (defvar-local outline-heading-alist ()
   "Alist associating a heading for every possible level.
 Each entry is of the form (HEADING . LEVEL).
@@ -1012,10 +1034,34 @@ outline--make-button-overlay
         (overlay-put o 'face (plist-get icon 'face))))
     o))
 
-(defun outline--insert-open-button ()
+(defun outline--make-margin-overlay (type)
+  (let ((o (seq-find (lambda (o)
+                       (overlay-get o 'outline-margin))
+                     (overlays-at (point)))))
+    (unless o
+      (setq o (make-overlay (point) (1+ (point))))
+      (overlay-put o 'follow-link 'mouse-face)
+      (overlay-put o 'mouse-face 'highlight)
+      (overlay-put o 'outline-margin t))
+    (let ((icon
+           (icon-elements (if (eq type 'close) 'outline-close 'outline-open)))
+          (inhibit-read-only t))
+      (overlay-put o 'before-string
+                   (propertize " " 'display `((margin left-margin)
+                                              ,(or (plist-get icon 'image)
+                                                   (plist-get icon 
'string))))))
+    o))
+
+(defun outline--insert-open-button (&optional margins-p)
   (with-silent-modifications
     (save-excursion
-        (beginning-of-line)
+      (beginning-of-line)
+      (if margins-p
+          (let ((o (outline--make-margin-overlay 'open)))
+            (overlay-put o 'help-echo "Click to hide")
+            (overlay-put o 'keymap
+                         (define-keymap
+                           "<mouse-2>" #'outline-hide-subtree)))
         (when (derived-mode-p 'special-mode)
           (let ((inhibit-read-only t))
             (insert "  ")
@@ -1025,12 +1071,18 @@ outline--insert-open-button
           (overlay-put o 'keymap
                        (define-keymap
                          "RET" #'outline-hide-subtree
-                         "<mouse-2>" #'outline-hide-subtree))))))
+                         "<mouse-2>" #'outline-hide-subtree)))))))
 
-(defun outline--insert-close-button ()
+(defun outline--insert-close-button (&optional margins-p)
   (with-silent-modifications
     (save-excursion
-        (beginning-of-line)
+      (beginning-of-line)
+      (if margins-p
+          (let ((o (outline--make-margin-overlay 'close)))
+            (overlay-put o 'help-echo "Click to show")
+            (overlay-put o 'keymap
+                         (define-keymap
+                           "<mouse-2>" #'outline-show-subtree)))
         (when (derived-mode-p 'special-mode)
           (let ((inhibit-read-only t))
             (insert "  ")
@@ -1040,23 +1092,25 @@ outline--insert-close-button
           (overlay-put o 'keymap
                        (define-keymap
                          "RET" #'outline-show-subtree
-                         "<mouse-2>" #'outline-show-subtree))))))
+                         "<mouse-2>" #'outline-show-subtree)))))))
 
 (defun outline--fix-up-all-buttons (&optional from to)
-  (when (outline--use-buttons-p)
-    (when from
-      (save-excursion
-        (goto-char from)
-        (setq from (line-beginning-position))))
-    (outline-map-region
-     (lambda ()
-       (if (save-excursion
-             (outline-end-of-heading)
-             (seq-some (lambda (o) (eq (overlay-get o 'invisible) 'outline))
-                       (overlays-at (point))))
-           (outline--insert-close-button)
-         (outline--insert-open-button)))
-     (or from (point-min)) (or to (point-max)))))
+  (let ((buttons-p (outline--use-buttons-p))
+        (margins-p (outline--use-margins-p)))
+    (when (or buttons-p margins-p)
+      (when from
+        (save-excursion
+          (goto-char from)
+          (setq from (line-beginning-position))))
+      (outline-map-region
+       (lambda ()
+         (if (save-excursion
+               (outline-end-of-heading)
+               (seq-some (lambda (o) (eq (overlay-get o 'invisible) 'outline))
+                         (overlays-at (point))))
+             (outline--insert-close-button margins-p)
+           (outline--insert-open-button margins-p)))
+       (or from (point-min)) (or to (point-max))))))
 
 (define-obsolete-function-alias 'hide-subtree #'outline-hide-subtree "25.1")