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23.4.1 Mode Line Basics

The contents of each mode line are specified by the buffer-local variable mode-line-format (see Mode Line Top). 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.

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 Mode Line Variables), or any other data structures that affect how text is displayed (see 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. Other windows' mode lines appear in the face mode-line-inactive instead. See Faces.