Add an update function

Also remove forcibly redisplaing. Not sure it's necessary, so let's wait and
see if things break.

center-content.el

@@ -1,4 +1,4 @@
-;;; center-content.el --- Center buffer content horizontally and vertically -*- lexical-binding: t; -*-
+;;; center-content.el --- Center buffer content -*- lexical-binding: t; -*-
 
 ;;; Commentary:
 
@@ -9,13 +9,45 @@
 
 (require 'face-remap)
 
+(defgroup center-content ()
+  "Center buffer content."
+  :group 'convenience
+  :prefix "center-content-")
+
+(defcustom center-content-horizontal t
+  "Enable horizontal centering."
+  :type 'boolean
+  :group 'center-content)
+
+(defcustom center-content-vertical t
+  "Enable vertical centering."
+  :type 'boolean
+  :group 'center-content)
+
+(defcustom center-content-vertical-by-overlay nil
+  "Enable vertical by using overlays.
+
+  Using overlays for vertical centering may give some undesirable
+  effects on the first line. By default, vertical centering is achieved
+  by using setting `header-line-format'."
+  :type 'boolean
+  :group 'center-content)
+
+(defcustom center-content-hide-mode-line nil
+  "Hide mode-line when centering."
+  :type 'boolean
+  :group 'center-content)
+
 (defvar-local center-content--horizontal-overlay nil
   "Overlay used to add left margin for horizontal centering.")
 
-(defvar-local center-content--original-header-line-format nil
+(defvar-local center-content--vertical-overlay nil
+  "Overlay used to add top margin for vertical centering.")
+
+(defvar-local center-content--original-header-line-format header-line-format
   "Stores the original `header-line-format` to restore upon disabling.")
 
-(defvar-local center-content--original-mode-line-format nil
+(defvar-local center-content--original-mode-line-format mode-line-format
   "Stores the original `mode-line-format` to restore upon disabling.")
 
 (defvar-local center-content--header-line-face-remap-cookie nil
@@ -23,7 +55,7 @@
 
 (defun center-content--content-pixel-size ()
   "Return the size of the content of the visible text in pixels."
-  (window-text-pixel-size (selected-window) (window-start) (window-end)))
+  (window-text-pixel-size (selected-window) (window-start) (window-end nil t)))
 
 (defun center-content--calculate-left-margin (content-width)
   "Calculate the left margin relative to CONTENT-WIDTH."
@@ -37,29 +69,44 @@
         (line-height (default-line-height)))
     (/ (max 0 (- window-height content-height)) (* 2 line-height))))
 
-(defun center-content--enable ()
-  "Enable horizontal and vertical centering of content."
-  (let* ((content-size (center-content--content-pixel-size))
-         (left-margin (center-content--calculate-left-margin (car content-size)))
-         (top-margin (center-content--calculate-top-margin (cdr content-size))))
-    ;; Apply horizontal centering using overlay
-    (let ((overlay (make-overlay (point-min) (point-max))))
-      (overlay-put overlay 'line-prefix
-                   (propertize " " 'display `(space :width ,left-margin)))
-      (setq center-content--horizontal-overlay overlay))
-    ;; Apply vertical centering using header-line-format
+(defun center-content--horizontal (content-width)
+  "Add overlay for horizontal centering relative to CONTENT-WIDTH."
+  (let* ((left-margin (center-content--calculate-left-margin content-width))
+         (overlay (make-overlay (point-min) (point-max)))
+         (space (propertize " " 'display `(space :width ,left-margin))))
+    (overlay-put overlay 'line-prefix space)
+    (setq center-content--horizontal-overlay overlay)))
+
+(defun center-content--vertical (content-height)
+  "Add overlay for vertical centering relative to CONTENT-HEIGHT."
+  (let* ((top-margin (center-content--calculate-top-margin content-height)))
     ;; Save and remap the header-line face to match the default face
-    (setq center-content--original-header-line-format header-line-format)
     (setq center-content--header-line-face-remap-cookie
           (face-remap-add-relative 'header-line 'default))
-    (setq header-line-format
-          (propertize " " 'display `(height ,top-margin)))
-    ;; Save and hide the mode line
+    (setq header-line-format (propertize " " 'display `(height ,top-margin)))))
+
+(defun center-content--vertical-by-overlay (content-height)
+  "Add overlay for vertical centering relative to CONTENT-HEIGHT."
+  (let* ((top-margin (center-content--calculate-top-margin content-height))
+         (overlay (make-overlay  (window-start) (window-start) nil t t))
+         (space (propertize " " 'display `(space :width 0 :height ,top-margin))))
+    (overlay-put overlay 'intangible t)
+    (overlay-put overlay 'before-string space)
+    (setq center-content--vertical-overlay overlay)))
+
+(defun center-content--enable ()
+  "Enable horizontal and vertical centering of content."
+  (setq center-content--original-header-line-format header-line-format)
   (setq center-content--original-mode-line-format mode-line-format)
-    (setq mode-line-format nil)
-    ;; Force redisplay
-    (force-mode-line-update)
-    (redisplay)))
+  (let* ((content-size (center-content--content-pixel-size)))
+    (when center-content-horizontal
+      (center-content--horizontal (car content-size)))
+    (when center-content-vertical
+      (if center-content-vertical-by-overlay
+          (center-content--vertical-by-overlay (cdr content-size))
+        (center-content--vertical (cdr content-size))))
+    (when center-content-hide-mode-line
+      (setq mode-line-format nil))))
 
 (defun center-content--disable ()
   "Disable centering of content."
@@ -67,30 +114,35 @@
   (when (overlayp center-content--horizontal-overlay)
     (delete-overlay center-content--horizontal-overlay)
     (setq center-content--horizontal-overlay nil))
+  ;; Remove vertical centering overlay
+  (when (overlayp center-content--vertical-overlay)
+    (delete-overlay center-content--vertical-overlay)
+    (setq center-content--vertical-overlay nil))
   ;; Restore the header-line face remapping
   (when center-content--header-line-face-remap-cookie
     (face-remap-remove-relative center-content--header-line-face-remap-cookie)
     (setq center-content--header-line-face-remap-cookie nil))
   ;; Restore the header-line-format
   (setq header-line-format center-content--original-header-line-format)
-  (setq center-content--original-mode-line-format nil)
   ;; Restore the original mode-line-format
-  (setq mode-line-format center-content--original-mode-line-format)
-  (setq center-content--original-header-line-format nil)
-  ;; Force redisplay
-  (force-mode-line-update)
-  (redisplay))
+  (setq mode-line-format center-content--original-mode-line-format))
+
+(defun center-content-update ()
+  "Updates the centering if `center-content-mode' is enabled."
+  (interactive)
+  (when (bound-and-true-p center-content-mode)
+    (center-content--disable)
+    (center-content--enable)))
 
 ;;;###autoload
 (define-minor-mode center-content-mode
   "Toggle horizontal and vertical centering of buffer content.
 
-When enabled, the content of the buffer is centered both
-horizontally and vertically within the window. The mode line is
-hidden for a clean appearance."
+When enabled, the content of the buffer is centered both horizontally
+and vertically within the window."
   :init-value nil
   :global nil
-  :lighter nil  ;; Remove the lighter to keep the mode line clean
+  :lighter " Center"
   (if center-content-mode
       (center-content--enable)
     (center-content--disable)))