focus-follows-mouse

focus-follows-mouse is a variable defined in `C source code'.
Its value is

nil

Documentation:
Non-nil if window system changes focus when you move the mouse.
You should set this variable to tell Emacs how your window manager
handles focus, since there is no way in general for Emacs to find out
automatically. See also `mouse-autoselect-window'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 20.3 of Emacs.