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

[Alan Mackenzie]

Hello, Emacs.

In the emacs-28 branch (also master), in a GUI session:

(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.

-- 
Alan Mackenzie (Nuremberg, Germany).