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24.4.1 Mode Line Basics

The contents of each mode line are specified by the buffer-local variable mode-line-format (see The Top Level of Mode Line Control). This variable holds a mode line construct: a template that controls what is displayed on the buffer’s mode line. The value of header-line-format specifies the buffer’s header line in the same way. All windows for the same buffer use the same mode-line-format and header-line-format unless a mode-line-format or header-line-format parameter has been specified for that window (see Window Parameters).

For efficiency, Emacs does not continuously recompute each window’s mode line and header line. It does so when circumstances appear to call for it—for instance, if you change the window configuration, switch buffers, narrow or widen the buffer, scroll, or modify the buffer. If you alter any of the variables referenced by mode-line-format or header-line-format (see Variables Used in the Mode Line), or any other data structures that affect how text is displayed (see Emacs Display), you should use the function force-mode-line-update to update the display.

Function: force-mode-line-update &optional all ¶

This function forces Emacs to update the current buffer’s mode line and header line, based on the latest values of all relevant variables, during its next redisplay cycle. If the optional argument all is non-nil, it forces an update for all mode lines and header lines.

This function also forces an update of the menu bar and frame title.

The selected window’s mode line is usually displayed in a different color using the face mode-line-active. Other windows’ mode lines appear in the face mode-line-inactive instead. See Faces.

Function: mode-line-window-selected-p ¶

If you want to have more extensive differences between the mode lines in selected and non-selected windows, you can use this predicate in an :eval construct. For instance, if you want to display the buffer name in bold in selected windows, but in italics in the other windows, you can say something like:

(setq-default
 mode-line-buffer-identification
 '(:eval (propertize "%12b"
		     'face (if (mode-line-window-selected-p)
			       'bold
			     'italic))))

Some modes put a lot of data in the mode line, pushing elements at the end of the mode line off to the right. Emacs can “compress” the mode line if the mode-line-compact variable is non-nil by turning stretches of spaces into a single space. If this variable is long, this is only done when the mode line is wider than the currently selected window. (This computation is approximate, based on the number of characters, and not their displayed width.) This variable can be buffer-local to only compress mode-lines in certain buffers.