bug#51632: Emacs 28: posn-at-x-y fails to take a tab line into account.

[martin rudalics]

> (i) emacs -Q
> (ii) C-u C-h i <and info file> RET
> (iii) M-: (point) ; Note this value
> (iv) M-: (posn-at-x-y 0 18) ; Where 18 is the height of your default
>    font.  Note that the second element of the result matches that from
>    (iii)
>
> (v) M-x tab-bar-mode RET
> (vi) M-: (posn-at-x-y 0 18) ; This returns the same as (iv).
>
> Because of the tab line, (vi) should return the symbol `header-line' in
> its second element, not the same as (iv).  This is a bug.

I'm not sure I understand.  'tab-bar-mode' puts a tab bar on top of the
frame.  How is this related to 'posn-at-x-y' which, by default, deals
with the selected window?

martin