Just tried opening Cura on my main iMac screen and everything seems to be working normally. Running Cura 4.9.1 and macOS Big Sur 11.4 on a 2017 Intel iMac with a second monitor attached. I usually have Cura on the second monitor. I was also unable to select a model by clicking on it. "Select all" works but I was not able to rotate or reposition any of the selected items. I could still right click and set up to slice multiple copies of whatever was selected.