MLScroll - Modeline Scrollbar for Emacs

A lightweight scrollbar for the Emacs mode line.

For graphical window systems:

and terminals too:

Why?

Emacs has so many great ways to navigate, I really only ever used scroll-bars as a visual indication of position and file size. But recently, to save space, I added (scroll-bar-mode -1) to my init. Immediately I missed having that information at a glance. A percentage in the mode line (like 25%) is not very glanceable, and also gives no information about the currently visible content relative to the total line length (i.e. from the length of the bar). I wanted something very fast and very lightweight, plus I'm not so into rainbow cats. Hence MLScroll.

Info

• Uses specified space for drawing (only 3 variable-width spaces, actually) for lightning-fast text-based mode line scroll.
• The bar lengths convey the number of lines above/visible/below window. Computes line numbers with caching for performance.
• Works in terminals! Terminal graphics are more granular (the minimum "pixel" is a character wide). You might want to increase mlscroll-width-chars there.
• In graphical Emacs, you can interact with the mode line scroll bar — click, click + drag, and wheel-scroll a full window height at a time.
• Clicking on an inactive window's mode line scroll bar scrolls it, but does not activate that window.

Installing

Get it from MELPA, and arrange to have:

(mlscroll-mode 1)

called at init time (or whenever you are feeling scrolly). Toggle on or off anytime.

Example for use-package:

(use-package mlscroll
  :ensure t
  :config
  (setq mlscroll-shortfun-min-width 11) ; truncate which-func
  (mlscroll-mode 1))

Alternatively, if you start emacs using the command line --daemon command (see below):

(use-package mlscroll
   :ensure t
   :hook (server-after-make-frame . mlscroll-mode))

Setup

Customization

Use M-x customize-group mlscroll to change background colors, overall scroll bar width, minimum current "thumb" width, border size, and other settings.

Position

By default, MLScroll disables the XX% position mode line indication, and puts itself in the mode line variable mode-line-end-spaces, prepending a spacer to right-align itself. Optionally, it can instead replace the XX% indicator (or be placed anywhere in your mode line). See mlscroll-alter-percent-position and mlscroll-right-align.

Which-function mode

The builtin which-function-mode by default puts [full-function-name] on the right side of the mode line. With long names and the default placement, this can push the MLScroll bar partially or fully off the mode line. Set mlscroll-shortfun-min-width to a minimum width, and MLScroll will truncate the which-function name to at least that many trailing characters (e.g. […function-name] or similar), to make room for the scrollbar. The amount of truncation can be adjusted using mlscroll-shortfun-extra-width.

Integrating with the mode-line

Note that MLScroll is most visually compatible with "plain" mode line formats that don't use :box bordering. It will warn you if you try to use a border with a :box-full format enabled. It also doesn't inherit :underline and :overline mode line properties unless mlscroll-border is set to 0 (these don't work with the combination of specified space and :box).

See the suggestions for configuring moody for some config ideas.

For users of modus-themes, (setq modus-themes-mode-line '(moody borderless)) is recommended, or, in more recent versions (>v4):

(setq modus-themes-common-palette-overrides
      '((border-mode-line-active unspecified)
        (border-mode-line-inactive unspecified)))

Scroll-bar width: Lines vs. Characters

The MLScroll bar widths are based on the number of lines visible in the window (+ lines before and after it). The normal scroll bar is based on characters shown. Both tend to change as very long/wrapped lines come into view, but in the opposite sense: MLScroll sees fewer lines shown and shrinks the current thumb; the default scrollbar sees many characters come in view, and grows it. I find the lines approach to be more sensible, and it has the advantage that with truncate-lines on, the thumb doesn't change size as you scroll. If you'd like to see the difference, evaluate (insert (make-string 5000 ?a) "\n") in the *scratch* buffer amidst other normal text, and scroll through before and after toggle-truncate-lines. On the other hand, highly folded documents like org-mode docs will show a changing "thumb size" as you scroll through, as the current window could contain many (hidden) lines. I find that pretty convenient actually.

FAQ's

• How does it work? MLScroll places itself by default in mode-line-end-spaces, and uses a right-aligned space to align it at the end of the modeline. For drawing the bars themselves, it uses three "specified spaces".

• MLScroll doesn't work with my fancy mode line mode! It should work automatically for simple mode lines that end in mode-line-end-spaces. If you prefer, set mlscroll-right-align to nil and mlscroll-alter-percent-position to 'replace to put it in place of the XX% percentage indicator. Otherwise, e.g. if you have a highly customized or pre-packaged mode line, you'll need to find somewhere to put MLScroll. A general recipe is to:

  1. Set mlscroll-right-align to nil.
  2. Set the mode line scroller into the relevant mode-line variable directly yourself, like so: (setq fancy-mode-line-variable-of-some-kind '(:eval (mlscroll-mode-line)).
  3. Alternatively, if you didn't design your mode line yourself or find this too complicated, ask whoever did to support MLScroll.

• MLScroll starts off very small when I start an emacs session using --daemon. For graphical windows, MLScroll needs to know the width of font characters in the mode line (or at least the default font) to draw a pixel-perfect bar. Since --daemon doesn't create a frame or know anything about the font widths, loading MLScroll directly under a --daemon session misreports the font width as 1 pixel, leading to a very small scroller bar. The solution is either to abandon --daemon in favor of (server-start) in your init file, or arrange for MLScroll to be initialized later, after a frame is created, ala:

  (use-package mlscroll
    :hook (server-after-make-frame . mlscroll-mode))
  

• How can I customize MLScroll? M-x customize-group mlscroll [Ret].

• I want to use MLScroll with different themes throughout the day, what should I do? This should happen automatically since v0.2. Since the MLScroll "thumb" color defaults to the foreground color of the scroll-bar face, you might configure that face for your theme, rather than mlscroll-in-color directly.

• I get a message about :box disabling my MLScroll border: MLScroll uses the :box attribute to draw border (with the help of :inverse-video). If your normal mode line face already has a :box property, this will interfere and cause the left/right border to show up. If you want a border to make your MLScroll less tall, consider disabling the :box property on faces 'mode-line and 'mode-line-inactive (M-x customized-group mode-line-faces [Ret]). See issue. For users of modus-themes, see above.

Other tips

MLScroll takes up a decent (configurable) chunk of your mode line. To save space for it even when the window is somewhat narrow, I use:

• minions to hide all minor-modes under a nice menu.
• cyphejor to shorten the names of major mode using single-char emoji and greek characters.
• (setq mlscroll-shortfun-min-width 11) to trim down the which-function name as needed.
• removal of mule-info, and a trim of all double-spaces anywhere in the mode line format to a single space:

  (setq-default
   mode-line-format ;less space, no MULE
   (cl-nsubst-if " " (lambda (x) (and (stringp x) (string-blank-p x) (> (length x) 1)))
		 (remove 'mode-line-mule-info mode-line-format))))