Marginalized atx headers

- markdown-marginalize-header

  Turns this header marginalization on/off. When it's on, the mode
  will configure the window appropriately. Off by default.

- markdown-marginalize-header-in-header-face

  Whether the marginalized header marks should themselves be displayed
  in the header face. Off by default.

  If you turn it on, you will probably also need to adjust your margin
  size manually using markdown-marginalize-header-margin-size, or else
  the header-faced markup characters will not fit in the window margin
  that was sized based on text-faced characters.

- markdown-marginalize-header-margin-size

  Size of the left margin, by default, seven. You will need to adjust
  this manually if you want your marginalized headers to be rendered
  in header faces.

markdown-mode.el

@@ -1087,6 +1087,45 @@ promotion and demotion functions."
   :group 'markdown
   :type 'boolean)
 
+(defcustom markdown-marginalize-header nil
+  "When non-nil, put opening atx header markup in a left margin.
+
+This setting goes well with `markdown-asymmetric-header'. But
+sadly it conflicts with `linum-mode' since they both use the
+same margin."
+  :group 'markdown
+  :type 'boolean)
+
+(defconst markdown-marginalize-margin-chars-needed 7
+  "Number of characters needed for marginalized headers.
+
+This is distinct from the size of the left margin, as measured in
+a number of text-sized characters, which is needed for rendering
+atx headers in header-sized characters into that margin.")
+
+(defcustom markdown-marginalize-header-in-header-face nil
+  "When non-nil, render marginalized atx header markup in header face.
+
+If enabled, and if your header face is larger than the text face,
+then you will likely need also to customize
+`markdown-marginalize-header-margin-size' to ensure it is big
+enough to show the header marks. That is because this mode is not
+smart enough to calculate the margin needed in text-sized
+characters for header-sized strings. Sorry!"
+  :group 'markdown
+  :type 'boolean)
+
+(defcustom markdown-marginalize-header-margin-size 7
+  "Size of the window left margin, for marginalized atx headers.
+
+The default, 7, is a suitable number if
+`markdown-marginalize-header-in-header-face' is nil. If it is
+non-nil, then you may need to experiment with increasing this
+number to a larger value, like 10 or 20, to ensure the margin is
+large enough to render your marginalized headers."
+  :group 'markdown
+  :type 'integer)
+
 (defcustom markdown-asymmetric-header nil
   "Determines if atx header style will be asymmetric.
 Set to a non-nil value to use asymmetric header styling, placing
@@ -3988,15 +4027,43 @@ Group 7: closing filename delimiter"
 
 ;;; Markdown Font Fontification Functions =====================================
 
+(defun generate-margin-string (margin-width heading-char-count)
+  "Generate string to place in the left margin.
+
+The string has length MARGIN-WIDTH - 1 and has HEADING-CHAR-COUNT
+heading characters."
+  (let* ((margin-left-space-count (- margin-width heading-char-count))
+         (margin-string (concat (make-string margin-left-space-count ? )
+                                (make-string heading-char-count ?#))))
+    margin-string))
+
+(defun markdown-marginalize-update-current ()
+  "Update the window configuration to create a left margin."
+  (set-window-margins nil markdown-marginalize-header-margin-size))
+
 (defun markdown-fontify-headings (last)
   "Add text properties to headings from point to LAST."
   (when (markdown-match-propertized-text 'markdown-heading last)
     (let* ((level (markdown-outline-level))
            (heading-face
             (intern (format "markdown-header-face-%d" level)))
            (heading-props `(face ,heading-face))
-           (markup-props `(face markdown-header-delimiter-face
-                                ,@(when markdown-hide-markup `(display ""))))
+           (markup-props
+            (if markdown-marginalize-header
+                ;; plist of text property for atx header
+                ;; when markdown-marginalize-header
+                `(
+                  ;; maybe render characters in the header face
+                  ,@(when markdown-marginalize-header-in-header-face
+                      `(face ,heading-face))
+                  ;; definitely place in the margin
+                  display ((margin left-margin)
+                           ,(generate-margin-string
+                             markdown-marginalize-margin-chars-needed level)))
+
+              ;; plist defining text properties of normal atx headers
+              `(face markdown-header-delimiter-face
+                     ,@(when markdown-hide-markup `(display "")))))
            (rule-props `(face markdown-header-rule-face
                               ,@(when markdown-hide-markup `(display "")))))
       (if (match-end 1)
@@ -9050,6 +9117,11 @@ position."
             #'markdown--edit-indirect-after-commit-function
             nil 'local)
 
+  (when markdown-marginalize-header
+    (add-hook 'window-configuration-change-hook
+              'markdown-marginalize-update-current nil t)
+    )
+
   ;; add live preview export hook
   (add-hook 'after-save-hook #'markdown-live-preview-if-markdown t t)
   (add-hook 'kill-buffer-hook #'markdown-live-preview-remove-on-kill t t))