So, I just installed ModernMix, and it's great, but there are issues with it (it's still in beta, so we'll see if they can fix this somehow.  Although I can finally resize a Modern UI window that plays video, if I switch focus to another Modern UI app, the media stops playing.  The only way to have 2 windows open and have it keep playing is to dock the media playing application.  Docking one app causes the 2nd app to immediately revert to the standard fullscreen (or 3/4, 5/8, whatever it is) mode.