[samatman]

In one way, 'focus' already follows mouse in macOS. I can mouse over to a window and start scrolling, so mouse focus follows the mouse.

And we don't want "OS focus" to follow the mouse! That would pop windows to the front, change the menu bar, and other unfortunate consequences: if I were mousing up toward the menu and accidentally crossed another window, I'd get the wrong menu.

What we want is for keyboard focus to also follow the mouse, and there's not actually any inherent reason why this can't happen. It's just that they're tightly coupled by the implementation, to the point where an extension can't un-couple them.

The lack of (keyboard) focus-follows-mouse is the big fly in my macOS ointment, it annoys me whenever I'm on my widescreen, which is at least half of the time. I do think it would be possible to implement a usable f-f-m which still worked the way macOS does with respect to menus, which windows get priority, and so on.

It would require uncoupling some things under the hood, which would probably affect the API in an incompatible way, so I don't expect to ever see it happen unfortunately.