A window can have a header line at the top, just as it can have
a mode line at the bottom. The header line feature works just like the
mode line feature, except that it's controlled by
This variable, local in every buffer, specifies how to display the header line, for windows displaying the buffer. The format of the value is the same as for
mode-line-format(see Mode Line Data). It is normally
nil, so that ordinary buffers have no header line.
A window that is just one line tall never displays a header line. A window that is two lines tall cannot display both a mode line and a header line at once; if it has a mode line, then it does not display a header line.