I'm having a strange problem using AirPlay from a MacBook Pro running OS X 10.14.6 to Roku Streaming Stick+. I am trying to stream a livestream that displays within a window on a webpage (the webpage is itself displayed in a tab in Safari). Screen mirroring works fine. So I can mirror my entire desktop (using the AirPlay icon in the OS X top menu bar), and see the webpage displayed on the TV, and the audio streams as well. However the livestream window on the web page has its own AirPlay icon within the streaming window itself. And I would prefer to AirPlay just that stream, so that I could see it on the TV, and switch to another window on my desktop while the stream keeps playing in the background on my computer, and thus also on my TV. That would allow me to view a related web page containing text that goes along with what is happening in the livestream. Unfortunately, when I click on the Airplay icon within the livestream, as opposed to in the OS X top menubar, the only thing that comes up on the TV screen is the AirPlay menu, where I can access the Roku AirPlay and HomeKit settings. In other words, mirroring works, but streaming from a window doesn't. Thanks for any suggestions.