Like this, do you mean?
This is from the Windows 10 dedicated app in the Microsoft Store; if you cursor up from full screen, a white bar appears at the top, and from there, you can adjust the size of the window you want to watch.
There was actually video - the full picture shrunk into the window - when I took the screenshot, but NowTV is clever enough to hide it when you snapshot the screen, and I haven't got time at the moment to figure out how to defeat that. But nevermind, you can try it for yourselves and see that it works.
When you want to pick another programme to watch, you have to go back to fullscreen to select it, but once it's started, you can shrink it again.
I can't answer for how the Browser/Player combination works on this, but anyone with Windows 10 should be using the dedicated app instead anyway.
