Comment by PaulDavisThe1st

but raising the window under the mouse also, for "true" FFM, gives kbd focus to the raised window, which will in turn (via whatever GUI toolkit is being used) moved kbd focus to some widget in the window, if possible.

It is impossible to generalize about this under X Window, since the behavior is entirely controlled by the window manager, and their behavior (a) varies (b) is very customizable.