1 files changed, 78 insertions, 0 deletions
diff --git a/lisp/display-line-numbers.el b/lisp/display-line-numbers.el
index 860aa758bce..897a88398fd 100644
--- a/lisp/display-line-numbers.el
+++ b/lisp/display-line-numbers.el
@@ -108,6 +108,84 @@ the mode is on, set `display-line-numbers' directly."
 (define-globalized-minor-mode global-display-line-numbers-mode
   display-line-numbers-mode display-line-numbers--turn-on)
 
+
+
+;;;###autoload
+(defvar header-line-indent ""
+  "String to indent at the start if the header line.
+This is used in `header-line-indent-mode', and buffers that have
+this switched on should have a `header-line-format' that look like:
+
+  (\"\" header-line-indent THE-REST...)
+
+Also see `header-line-indent-width'.")
+
+;;;###autoload
+(defvar header-line-indent-width 0
+  "The width of the current line numbers displayed.
+This is updated when `header-line-indent-mode' is switched on.
+
+Also see `header-line-indent'.")
+
+(defun header-line-indent--line-number-width ()
+  "Return the width taken by `display-line-numbers' in the current buffer."
+  ;; line-number-display-width returns the value for the selected
+  ;; window, which might not be the window in which the current buffer
+  ;; is displayed.
+  (if (not display-line-numbers)
+      0
+    (let ((cbuf-window (get-buffer-window (current-buffer) t)))
+      (if (window-live-p cbuf-window)
+          (with-selected-window cbuf-window
+            (truncate (line-number-display-width 'columns)))
+        4))))
+
+(defun header-line-indent--watch-line-number-width (_window)
+  (let ((width (header-line-indent--line-number-width)))
+    (setq header-line-indent-width width)
+    (unless (= (length header-line-indent) width)
+      (setq header-line-indent (make-string width ?\s)))))
+
+(defun header-line-indent--window-scroll-function (window _start)
+  (let ((width (with-selected-window window
+                 (truncate (line-number-display-width 'columns)))))
+    (setq header-line-indent-width width)
+    (unless (= (length header-line-indent) width)
+      (setq header-line-indent (make-string width ?\s)))))
+
+;;;###autoload
+(define-minor-mode header-line-indent-mode
+  "Mode to indent the header line in `display-line-numbers-mode' buffers.
+This means that the header line will be kept indented so that it
+has blank space that's as wide as the displayed line numbers in
+the buffer.
+
+Buffers that have this switched on should have a
+`header-line-format' that look like:
+
+  (\"\" header-line-indent THE-REST...)
+
+The `header-line-indent-width' variable is also kept updated, and
+has the width of `header-line-format'.  This can be used, for
+instance, in `:align-to' specs, like:
+
+  (space :align-to (+ header-line-indent-width 10))"
+  :lighter nil
+  (if header-line-indent-mode
+      (progn
+        (setq-local header-line-indent ""
+                    header-line-indent-width 0)
+        (add-hook 'pre-redisplay-functions
+                  #'header-line-indent--watch-line-number-width nil t)
+        (add-hook 'window-scroll-functions
+                  #'header-line-indent--window-scroll-function nil t))
+    (setq-local header-line-indent ""
+                header-line-indent-width 0)
+    (remove-hook 'pre-redisplay-functions
+                 #'header-line-indent--watch-line-number-width t)
+    (remove-hook 'window-scroll-functions
+                 #'header-line-indent--window-scroll-function t)))
+
 (provide 'display-line-numbers)
 
 ;;; display-line-numbers.el ends here