It isn’t so much a screenshot as it is the system suspending (parts of) apps that aren’t used or visible and aggressively compressing and decompressing memory.

It isn’t so much a screenshot as it is the system suspending (parts of) apps that aren’t used or visible and aggressively compressing and decompressing memory. So up until you tried to interact with it, it wasn’t frozen as far as the system was concerned, only then did it ran into an issue and did it convey that issue to you. I don’t know by heart what the last macOS (or even OS X) version and Xcode version is that would be supported by a decade old iMac, but I can’t imagine it being a pleasant experience.