24.4.8 Window Tab Lines

A window can have a tab line at the top. If both the tab line and header line are visible, the tab line appears above the header line. The tab line feature is controlled like the mode line feature, except that it’s controlled by tab-line-format. Unlike the mode line, the tab line is only expected to be used to display a list of tabs (see Tab Line in The GNU Emacs Manual) or the window tool bar (see Window Tool Bar in The GNU Emacs Manual):

Variable: tab-line-format ¶

This variable, local in every buffer, specifies how to display the tab line, for windows displaying the buffer. The format of the value is the same as for mode-line-format (see The Data Structure of the Mode Line). It is normally nil, so that ordinary buffers have no tab line.

Function: window-tab-line-height &optional window ¶

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