I’ve kinda been doing this for a while now, with the help of the floating overlays App, although the availability of supported floating Apps is limited at the moment.
Playstore Link :
https://play.google.com/store/apps/details?id=com.applay.overlay
Another one for Helliboard, tried Floris but spell checking is a must for me, once Floris has it, I may try things out.