Hello @heinoh ,
we are also testing the Realtime WYSIWYG Collaboration, XWiki 15.8 and we use different browser (Firefox, Chrome, Edge) to see if it works with any browser. But it seems to broken at the moment, not sure when it gets fixed.
In another post (Realtime WYSIWYG Editor Page locked - #16 by mflorea) @mflorea mentioned that they might work on it in the next months.