Re: [Emacs-diffs] master 29d1c72: Introduce new value t for compilation-context-lines to eliminate scrolling

[Alan Mackenzie]

Hello, Eli and Stefan.

On Sun, Aug 25, 2019 at 22:37:39 +0300, Eli Zaretskii wrote:
> > Date: Sun, 25 Aug 2019 19:06:37 +0000
> > From: Alan Mackenzie <address@hidden>
> > Cc: address@hidden

> > > 2- Why insert a prefix string (an "inserted arrow") instead of using
> > >    a "regular overwriting arrow"?

> > Because the overwriting arrow would obliterate the first two characters
> > of the file name.  I actually tried this first, and it wasn't
> > satisfactory.  This contrasts with another use of the overwriting arrow
> > in edebug, where (usually) only WS gets overwritten, and it is important
> > not to disturb the visible indentation.

> Did you consider to use the display margin instead?  You can have an
> overlay display in the margin by putting a 'display' property on the
> overlay string.  The advantage of using the margin is that it will
> keep the buffer text aligned.

OK, thanks for telling me about this.  I propose installing the following
patch which implements the arrow in the margin (and supplements my
earlier commit) into master.  Criticism, as always, will be welcomed.




diff --git a/lisp/progmodes/compile.el b/lisp/progmodes/compile.el
index 09188dc14b..931f66ef03 100644
--- a/lisp/progmodes/compile.el
+++ b/lisp/progmodes/compile.el
@@ -2602,45 +2602,70 @@ compilation-set-window
                               (point)))
          (set-window-point w mk))))
 
-(defvar-local overlay-arrow-overlay nil
+(defvar-local compilation-arrow-overlay nil
   "Overlay with the before-string property of `overlay-arrow-string'.
 
 When non-nil, this overlay causes redisplay to display `overlay-arrow-string'
 at the overlay's start position.")
 
+(defvar compilation-margin-string "=>"
+  "The string which will appear in the margin in compilation mode.
+This must be two characters long; there should be no need to
+change the default.")
+(put-text-property 0 2 'face 'default compilation-margin-string)
+
+(defvar compilation-dummy-string ">"
+  "A string which is only a placeholder for compilation-margin-string.
+It's actual value is never used, but must be one character long.")
+(put-text-property 0 1 'display
+                   `((margin left-margin) ,compilation-margin-string)
+                   compilation-dummy-string)
+
+(defun compilation-set-up-arrow-spec-in-margin ()
+  "Set up compilation-arrow-overlay to display as an arrow in a margin."
+  (setq overlay-arrow-string "")
+  (overlay-put compilation-arrow-overlay
+               'before-string compilation-dummy-string)
+  (set-window-margins (selected-window) 2))
+
+(defun compilation-tear-down-arrow-spec-in-margin ()
+  "Restore compilation-arrow-overlay to not using the margin, which is 
removed."
+  (overlay-put compilation-arrow-overlay 'before-string nil)
+  (delete-overlay compilation-arrow-overlay)
+  (setq compilation-arrow-overlay nil)
+  (set-window-margins (selected-window) 0))
+
 (defun compilation-set-overlay-arrow (w)
   "Set up, or switch off, the overlay-arrow for window W."
-  (with-current-buffer (window-buffer w)
-    (if (and (eq compilation-context-lines t)
-             (equal (car (window-fringes w)) 0)) ; No left fringe
-        ;; Insert a "=>" before-string overlay at the beginning of the
-        ;; line pointed to by `overlay-arrow-position'.
-        (cond
-         ((overlayp overlay-arrow-overlay)
-          (when (not (eq (overlay-start overlay-arrow-overlay)
-                        overlay-arrow-position))
-           (if overlay-arrow-position
-                (progn
-                 (move-overlay overlay-arrow-overlay
-                               overlay-arrow-position overlay-arrow-position)
-                  (setq overlay-arrow-string "=>")
-                  (overlay-put overlay-arrow-overlay
-                               'before-string overlay-arrow-string))
-             (delete-overlay overlay-arrow-overlay)
-             (setq overlay-arrow-overlay nil))))
-
-         (overlay-arrow-position
-          (setq overlay-arrow-overlay
-               (make-overlay overlay-arrow-position overlay-arrow-position))
-          (setq overlay-arrow-string "=>")
-          (overlay-put overlay-arrow-overlay 'before-string 
overlay-arrow-string)))
-
-      ;; `compilation-context-lines' isn't t, or we've got a left
-      ;; fringe, so remove any overlay arrow.
-      (when (overlayp overlay-arrow-overlay)
-        (setq overlay-arrow-string "")
-        (delete-overlay overlay-arrow-overlay)
-        (setq overlay-arrow-overlay nil)))))
+  (with-selected-window w        ; So the later `goto-char' will work.
+    (with-current-buffer (window-buffer w)
+      (if (and (eq compilation-context-lines t)
+               (equal (car (window-fringes w)) 0)) ; No left fringe
+          ;; Insert a before-string overlay at the beginning of the line
+          ;; pointed to by `overlay-arrow-position', such that it will
+          ;; display in a 2-character margin.
+          (progn
+            (cond
+             ((overlayp compilation-arrow-overlay)
+              (when (not (eq (overlay-start compilation-arrow-overlay)
+                            overlay-arrow-position))
+               (if overlay-arrow-position
+                   (move-overlay compilation-arrow-overlay
+                                 overlay-arrow-position overlay-arrow-position)
+                  (compilation-tear-down-arrow-spec-in-margin))))
+
+             (overlay-arrow-position
+              (setq compilation-arrow-overlay
+                   (make-overlay overlay-arrow-position 
overlay-arrow-position))
+              (compilation-set-up-arrow-spec-in-margin)))
+            ;; Ensure that the "=>" remains in the window by causing
+            ;; the window to be scrolled, if needed.
+            (goto-char (overlay-start compilation-arrow-overlay)))
+
+        ;; `compilation-context-lines' isn't t, or we've got a left
+        ;; fringe, so remove any overlay arrow.
+        (when (overlayp compilation-arrow-overlay)
+          (compilation-tear-down-arrow-spec-in-margin))))))
 
 (defvar next-error-highlight-timer)
 

-- 
Alan Mackenzie (Nuremberg, Germany).