I thought I should reply directly since I did some tests and don’t want to confuse the other users.
Since I have a dual monitor setup attached to my MacBook Pro, I opened the project as usual and the Dorico projected window opened on the attached monitor. To test a new theory I moved the project window to the MacBook Pro and tried with the mixer open while using the Play window and the Write window - it worked fine for both. Somehow at first the dual monitor was the problem but as soon as I moved to just the MacBook Pro I was OK. I took it one step further and moved the project window to the attached monitor and left the mixer on the MacBook Pro - Again it worked fine. It looks like now the project knows that I am using a dual monitor set up. I saved the file, closed Dorico, reopened the newly saved file - Play and Write windows worked fine with the mixer open.
One more step, I opened another project that gave me the same problem with the dual monitor when using the Play window with the mixer open it freezes. I moved the main Dorico window to the MacBook Pro and there it worked fine for both Write and Play windows but as soon as I moved the project window to the attached monitor with the Play window active and the mixer opened it Froze again. So the file didn’t work the same way as with the previous test. Both projects are using NotePerformer. Both programs are updated to current, except didn’t update to the new Mac OS currently using Catalina 10.15.7. MacBook Pro 2.9 GHz 6-core i9. Memory 32 Gb 2400 MHz DDR4. Graphics Radeon Pro Vega 20 4 GB and Intel UHD Graphics 630 1536 MB.
This is not an emergency situation as I just close the mixer window and continue working. Again I’m a big fan of Dorico.