> I'm not sure I follow: it is your code in
> tab-line-tab-name-format-default that applies the tab-line-highlight
> as mouse-face to the button:
>
>     (apply 'propertize
>            (concat (propertize name
>                                'keymap tab-line-tab-map
>                                ;; Don't turn mouse-1 into mouse-2 (bug#49247)
>                                'follow-link 'ignore)
>                    (or (and (or buffer-p (assq 'buffer tab) (assq 'close tab))
>                             tab-line-close-button-show
>                             (not (eq tab-line-close-button-show
>                                      (if selected-p 'non-selected 'selected)))
>                             tab-line-close-button)
>                        ""))
>            `(
>              tab ,tab
>              ,@(if selected-p '(selected t))
>              face ,face
>              mouse-face tab-line-highlight))))
>
> You should be able not to do that for the close button.  Or am I
> missing something?

It's possible to put properties on the close button only, but wouldn't this be
too ugly?  This means that hovering the mouse over the tab name will highlight only
the tab name without highlighting the close button.  And hovering the mouse
over the close button will highlight only the close button without highlighting
the tab name.  So highlighting will be separate for the close button and
the rest of the tab with such patch: