bug#37807: 27.0.50; Mouse events assigned to header line while pointer not in header line

[Eli Zaretskii]

> From: cpardo@imayhem.com
> Date: Fri, 18 Oct 2019 14:06:40 +0200
> 
> On a window with header line (package-list-packages), move the
> mouse pointer to the line inmediately below the header line. Move the
> mouse wheel, and get the message:
> 
> <header-line> <mouse-5> is undefined
> 
> Also, clickable items on the header line are highlighted as if the mouse
> was over them when the mouse is on the line below.
> 
> Tested on X11 and win32.

Thanks, should be fixed now.

There's still a problem with header line and tab-line displayed on TTY
frames, but that one is deeper.  I'm working on that.