You don't have to target a button at all, that's quite silly. Aim at the window label. Labels have always been connoted as "plain text" with no actions associated, so they're quite intuituve for that purpose.
Of course! But when you're using an application with a fairly busy header bar (like Nautilus or Epiphany, which don't have any labels) it can be useful to just point at the first pixel you come across without having to aim for a blank interval between buttons.
