New option: diff-hl-fallback-to-margin

Resolves #212

Author: dgutov

README.md

@@ -18,6 +18,9 @@ The package also contains auxiliary modes:
 * `diff-hl-dired-mode` provides similar functionality in Dired.
 * `diff-hl-margin-mode` changes the highlighting function to
   use the margin instead of the fringe.
+* But if you use a non-graphical terminal, the package will fall back to using
+  the margins anyway, as long as `diff-hl-fallback-to-margin` is non-nil and the
+  margin width is non-zero.
 * `diff-hl-amend-mode` sets the reference revision to the one before
   recent one. Also, you could use `diff-hl-set-reference-rev` to set
   it to any revision, see its docstring for details.

diff-hl-margin.el

@@ -1,6 +1,6 @@
 ;;; diff-hl-margin.el --- Highlight buffer changes on margins -*- lexical-binding: t -*-
 
-;; Copyright (C) 2012-2017  Free Software Foundation, Inc.
+;; Copyright (C) 2012-2025  Free Software Foundation, Inc.
 
 ;; This file is part of GNU Emacs.
 
@@ -147,6 +147,7 @@ You probably shouldn't use this function directly."
                         ,(propertize char 'face
                                      (intern (format "diff-hl-margin-%s" type)))))))))
 
+;;;###autoload
 (defun diff-hl-highlight-on-margin (ovl type _shape)
   (let ((spec (cdr (assoc (cons type diff-hl-side)
                           (diff-hl-margin-spec-cache)))))

diff-hl.el

@@ -1,6 +1,6 @@
 ;;; diff-hl.el --- Highlight uncommitted changes using VC -*- lexical-binding: t -*-
 
-;; Copyright (C) 2012-2024  Free Software Foundation, Inc.
+;; Copyright (C) 2012-2025  Free Software Foundation, Inc.
 
 ;; Author:   Dmitry Gutov <dmitry@gutov.dev>
 ;; URL:      https://github.com/dgutov/diff-hl
@@ -116,6 +116,14 @@
   :group 'diff-hl
   :type 'boolean)
 
+(defcustom diff-hl-fallback-to-margin t
+  "Non-nil to use margin instead of fringe on non-graphic displays.
+
+This requires the corresponding margin width to be >0 already, which is
+normally the case on such displays."
+  :group 'diff-hl
+  :type 'boolean)
+
 (defcustom diff-hl-highlight-function 'diff-hl-highlight-on-fringe
   "Function to highlight the current line. Its arguments are
   overlay, change type and position within a hunk."
@@ -500,8 +508,11 @@ It can be a relative expression as well, such as \"HEAD^\" with Git, or
     o))
 
 (defun diff-hl-highlight-on-fringe (ovl type shape)
-  (overlay-put ovl 'before-string (diff-hl-fringe-spec type shape
-                                                       diff-hl-side)))
+  (if (and diff-hl-fallback-to-margin
+           (not (display-graphic-p)))
+      (diff-hl-highlight-on-margin ovl type shape)
+    (overlay-put ovl 'before-string (diff-hl-fringe-spec type shape
+                                                         diff-hl-side))))
 
 (defun diff-hl-remove-overlays (&optional beg end)
   (save-restriction