With respect to 'X11', in 'Terminal' (yes - 'Terminal') enter ...
defaults write com.apple.x11 wm_ffm true
... <return>, if all ready open (running, launched, executed) quit 'X11', launch 'X11', etc.
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To disable the 'focus follows mouse' feature - in 'X11', in 'Terminal' enter ....
defaults write com.apple.x11 wm_ffm false
... <return>, if all ready open (running, launched, executed) quit 'X11', launch 'X11', etc.