23.4.7 Window Header Lines

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 header-line-format:

Variable: header-line-format

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.

Function: window-header-line-height &optional window

This function returns the height in pixels of window’s header line. window must be a live window, and defaults to the selected window.

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.