How to modify fringes only for magit mode?

Question

I have tried:

(defun my-change-fringes ()
  (set-window-fringes (selected-window) 20 0))

(add-hook 'magit-mode-hook 'my-change-fringes))

The problem seems to be that when I open a magit buffer for the first time, the selected window is for some reason the previous window.

Window focus probably hasn't updated yet when running the hook. Try resolving the window using `current-buffer` (the magit buffer in the case of the mode hook). Maybe `(get-buffer-window (current-buffer))` ? — ebpa, Feb 07 '19 at 20:00

phils · Accepted Answer · 2019-02-08T06:23:41.083

Note that if you call set-window-fringes in a mode hook, then your change will not be in effect if that existing magit buffer is then displayed in a different window.

The simplest solution to your problem is:

(defun my-magit-mode-hook ()
  "Custom `magit-mode' behaviours."
  (setq left-fringe-width 20
        right-fringe-width 0))

(add-hook 'magit-mode-hook 'my-magit-mode-hook)

A slightly more complex approach would be to use a buffer-local C-hv window-configuration-change-hook (see which for details) to make your set-window-fringes call:

(defun my-magit-window-config ()
  "Used in `window-configuration-change-hook' to configure fringes for Magit."
  (set-window-fringes nil 20 0))

(defun my-magit-mode-hook ()
  "Custom `magit-mode' behaviours."
  (add-hook 'window-configuration-change-hook
            'my-magit-window-config nil :local))

(add-hook 'magit-mode-hook 'my-magit-mode-hook)

And you can safely ignore the following entirely, but just in case it's of interest...

When the buffer changes in a given window it is still the same window object being used, and as such I had initially assumed that it would also be necessary to restore the default fringe sizes when a non-Magit buffer was displayed in a window.

However, set-window-buffer by default "resets WINDOW’s position, display margins, fringe widths, and scroll bar settings, based on the local variables in the specified buffer" (which is, of course, where the left-fringe-width and right-fringe-width buffer-local variables come into play).

Consequently the house-keeping I was attempting was entirely unnecessary (and moreover was undoubtedly problematic for anything else trying to set fringe widths).

So don't use the following code; but I'll leave it here in case it's of interest for any reason.

(defun my-magit-window-config ()
  "Used in `window-configuration-change-hook' to configure fringes for Magit."
  (let ((left (frame-parameter nil 'left-fringe))
        (right (frame-parameter nil 'right-fringe)))
    (cl-loop for win being the windows
             if (with-selected-window win (derived-mode-p 'magit-mode))
             do (set-window-fringes win 20 0)
             else do (set-window-fringes win left right))))

(add-hook 'window-configuration-change-hook 'my-magit-window-config)

Excellent explanation! The buffer-local hook setting is a trick I didn't know before. Now magit buffers show the section indicators without affecting other documents. Sweet! — ctietze, Dec 21 '21 at 10:37

How to modify fringes only for magit mode?

1 Answers1