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
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
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.