bug#51794: 29.0.50; invalid input events generated from a mouse click for the tab bar

[Eli Zaretskii]

> Cc: 51794@debbugs.gnu.org
> From: martin rudalics <rudalics@gmx.at>
> Date: Fri, 19 Nov 2021 10:26:52 +0100
> 
>  >> bug fix #50993 is the bug.  it made make_lispy_position skip setting
>  >> posn to the tab_bar if track_mouse is non-nil.  why track_mouse is nil
>  >> for some frames and not for others i didn't look into.
>  >> thoughts/comments?
>  >
>  > Please do look into that last part, because we need to understand the
>  > situations in which this happens well enough to devise a solution.  We
>  > don't want to revert that fix because it will re-introduce a larger
>  > problem.  Specifically, why does this happen to you only under those
>  > customizations and not in "emacs -Q"?
>  >
>  > Martin, any suggestions?
> 
> Maybe the attached patch helps.

Thanks.  Please install this on the emacs-28 branch, but please add
comments explaining why we need this special symbol.