Variable: tab-bar-border
tab-bar-border is a variable defined in xdisp.c.
Value
internal-border-width
Documentation
Border below tab-bar in pixels.
If an integer, use it as the height of the border.
If it is one of internal-border-width or border-width, use the
value of the corresponding frame parameter.
Otherwise, no border is added below the tab-bar.
Source Code
// Defined in /usr/src/emacs/src/xdisp.c
DEFVAR_LISP ("tab-bar-border", Vtab_bar_border,
doc: /* Border below tab-bar in pixels.
If an integer, use it as the height of the border.
If it is one of `internal-border-width' or `border-width', use the
value of the corresponding frame parameter.
Otherwise, no border is added below the tab-bar. */);