While streamlining my mode line, I wrote another little mode-line feature that I thought of ages ago — using the background of the mode-line to indicate the current position in the buffer. I didn’t like this enough to use it, but I thought I’d post it since it was a fun hack.

First, make sure the current mode line is kept:

(defvar tromey-real-mode-line-format mode-line-format)

Now, make a little function that format the mode line using the standard rules and then applies a property depending on the current position in the buffer:

(defun tromey-compute-mode-line ()
  (let* ((width (frame-width))
     (line (substring 
        (concat (format-mode-line tromey-real-mode-line-format)
            (make-string width ? ))
        0 width)))
    ;; Quote "%"s.
    (setq line
      (mapconcat (lambda (c)
               (if (eq c ?%)
               "%%"
             ;; It's absurd that we must wrap this.
             (make-string 1 c)))
             line ""))

    (let ((start (window-start))
      (end (or (window-end) (point))))
      (add-face-text-property (round (* (/ (float start)
                       (point-max))
                    (length line)))
                  (round (* (/ (float end)
                       (point-max))
                    (length line)))
                  'region nil line))
    line))

We have to do this funny wrapping and “%”-quoting business here because the :eval form returns a mode line format — not just text — and because the otherwise appealing :propertize form doesn’t allow computations.

Also, I’ve never understood why mapconcat can’t handle a character result from the map function.  Anybody?

Now set this to be the mode line:

(setq-default mode-line-format '((:eval (tromey-compute-mode-line))))

The function above changes the background of the mode line corresponding to the current window’s start and end positions.  So, for example, here we are in the middle of a buffer that is bigger than the window:

I left this on for a bit but found it too distracting.  If you like it, use it. You might like to remove the mode-line-position stuff from the mode line, as it seems redundant with the visual display.