DISABLE THE PRESENTER CONSOLE !

Slide Show -> Slide Show Setings -> Display -> Presenter console set to DISABLED

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Open libreoffice impress with presentation in some workspace

Connect projector, then do ./extend-right.sh or ./unify.sh

workspace+1 would be external screen (projector), so go to
libreoffie impress (in workspace), go into Slide Show using F5 key
move fullscreen slide show to external screen - $mod + shift + workspace+1