This request is a bit broad (in reference to the multiple clients being called out here) - but at least for part of it this functionality exists on Android right now. Here is a screenshot from my phone just now. Before anyone asks - I didn’t do any research to go back and see when this would have been added so YMMV here!
There are also ways to do this on web as well. Firefox has a built in function for picture in picture (just tested it on my device and it worked great). It doesn’t look like chrome has anything native, but I found extensions that will do it. As far as Windows - that is going to be a challenge right now and may not be something we consider working on for a while.
