Long time ago, that was actually very easy on Mac, via SIMBL (https://github.com/albertz/simbl) and Afloat (https://github.com/rwu823/afloat) and you could hack around using FScriptAnywhereSIMBL (https://github.com/albertz/FScriptAnywhereSIMBL) or Pyjector (https://github.com/albertz/Pyjector).

But that doesn't work anymore since a while (I guess due to SIP).