It is normally @code{nil}, so that ordinary buffers have no header line.
@end defvar
+The function @code{window-header-line-height} returns the height of
+the header line:
+
+@defun window-header-line-height &optional window
+Return the height of @var{window}'s header line, in pixels.
+@var{window} must be a live window. If @var{window} is @code{nil} or
+omitted, it will be the selected window.
+@end defun
+
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