>> 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.

martin