Musescore not alowed in Big Sur split screen?

I can split screen OK with other apps but when I try this with Firefox (I have a YouTube window open) and Musescore the Musescore app will not open and shows a message: "No available Windows" showing Musescore as a small tile. Clicking it does nothing. Any solutions to this? It is a very useful feature for transposing. PS, I have checked on the Apple forum for solutions with no success so far. Thanks.