Variable: header-line-format
header-line-format is a buffer-local variable defined in buffer.c.
Documentation
Analogous to mode-line-format, but controls the header line.
The header line appears, optionally, at the top of a window; the mode line appears at the bottom.
Also see header-line-indent-mode(var)/header-line-indent-mode(fun) if display-line-numbers-mode(var)/display-line-numbers-mode(fun) is
turned on and header-line text should be aligned with buffer text.
Probably introduced at or before Emacs version 21.1.
Source Code
// Defined in /usr/src/emacs/src/buffer.c
DEFVAR_PER_BUFFER ("header-line-format",
&BVAR (current_buffer, header_line_format),
Qnil,
doc: /* Analogous to `mode-line-format', but controls the header line.
The header line appears, optionally, at the top of a window; the mode
line appears at the bottom.
Also see `header-line-indent-mode' if `display-line-numbers-mode' is
turned on and header-line text should be aligned with buffer text. */);