Variable: tab-bar-close-button
tab-bar-close-button is a variable defined in tab-bar.el.gz.
Value
#(" x" 0 2 (close-tab t :help "Click to close tab"))
Documentation
Button for closing the clicked tab.
Source Code
;; Defined in /usr/src/emacs/lisp/tab-bar.el.gz
(defvar tab-bar-close-button
(propertize " x"
'close-tab t
:help "Click to close tab")
"Button for closing the clicked tab.")