bug#59947: 30.0.50; Underline of tab-bar face does not respect x-underline-at-descent-line

[Eli Zaretskii]

> From: Gabriel <gabriel376@hotmail.com>
> Date: Sat, 10 Dec 2022 15:22:20 -0300
> 
> Apparently, the underline property of tab-bar (and tab-bar-*) face does
> not respect the option x-underline-at-descent-line.  When
> x-underline-at-descent-line is set to t, the underline is displayed at
> descent level (below of bottom box), but in tab-bar face it's displayed
> at the baseline (above of bottom box).  See steps below and attached
> images for more information.

The x-underline-at-descent-line option is only supported for windows
that display buffers.  And the tab bar uses a pseudo-window that has
no buffer (like the tool bar in builds that don't have a
toolkit-provided tool bar).