28.13 Horizontal Scrolling

Horizontal scrolling means shifting the image in the window left or right by a specified multiple of the normal character width. Each window has a vertical scroll position, which is a number, never less than zero. It specifies how far to shift the contents left. Shifting the window contents left generally makes all or part of some characters disappear off the left, and all or part of some other characters appear at the right. The usual value is zero.

The horizontal scroll position is measured in units of the normal character width, which is the width of space in the default font. Thus, if the value is 5, that means the window contents are scrolled left by 5 times the normal character width. How many characters actually disappear off to the left depends on their width, and could vary from line to line.

Because we read from side to side in the "inner loop", and from top to bottom in the "outer loop", the effect of horizontal scrolling is not like that of textual or vertical scrolling. Textual scrolling involves selection of a portion of text to display, and vertical scrolling moves the window contents contiguously; but horizontal scrolling causes part of each line to go off screen.

Usually, no horizontal scrolling is in effect; then the leftmost column is at the left edge of the window. In this state, scrolling to the right is meaningless, since there is no data to the left of the edge to be revealed by it; so this is not allowed. Scrolling to the left is allowed; it scrolls the first columns of text off the edge of the window and can reveal additional columns on the right that were truncated before. Once a window has a nonzero amount of leftward horizontal scrolling, you can scroll it back to the right, but only so far as to reduce the net horizontal scroll to zero. There is no limit to how far left you can scroll, but eventually all the text will disappear off the left edge.

In Emacs 21, redisplay automatically alters the horizontal scrolling of a window as necessary to ensure that point is always visible, if automatic-hscrolling is set. However, you can still set the horizontal scrolling value explicitly. The value you specify serves as a lower bound for automatic scrolling, i.e. automatic scrolling will not scroll a window to a column less than the specified one.

Command: scroll-left &optional count

This function scrolls the selected window count columns to the left (or to the right if count is negative). The default for count is the window width, minus 2.

The return value is the total amount of leftward horizontal scrolling in effect after the change--just like the value returned by window-hscroll (below).

Command: scroll-right &optional count

This function scrolls the selected window count columns to the right (or to the left if count is negative). The default for count is the window width, minus 2.

The return value is the total amount of leftward horizontal scrolling in effect after the change--just like the value returned by window-hscroll (below).

Once you scroll a window as far right as it can go, back to its normal position where the total leftward scrolling is zero, attempts to scroll any farther right have no effect.

Function: window-hscroll &optional window

This function returns the total leftward horizontal scrolling of window---the number of columns by which the text in window is scrolled left past the left margin.

The value is never negative. It is zero when no horizontal scrolling has been done in window (which is usually the case).

If window is nil, the selected window is used.

(window-hscroll) => 0 (scroll-left 5) => 5 (window-hscroll) => 5

Function: set-window-hscroll window columns

This function sets the number of columns from the left margin that window is scrolled from the value of columns. The argument columns should be zero or positive; if not, it is taken as zero. Fractional values of columns are not supported at present.

The value returned is columns.

(set-window-hscroll (selected-window) 10) => 10

Here is how you can determine whether a given position position is off the screen due to horizontal scrolling:

(defun hscroll-on-screen (window position) (save-excursion (goto-char position) (and (>= (- (current-column) (window-hscroll window)) 0) (< (- (current-column) (window-hscroll window)) (window-width window)))))