In real, XWin-style focus-follows-mouse, the window you mouse over gets the focus, but, isn't pulled to the front. If you click, it gets the front and the focus.
I use Moom, a semi-tile snap-style window wrangler, and a widescreen with three panels. What I want to do is just slide the cursor over and start typing, but I have to remember to click, and often enough I just don't.
The One True Wiki has a nice discussion of the subject: https://wiki.c2.com/?FocusFollowsMouse