> I still see colleagues who have a separate IntelliJ window open for each project they need to work with because IntelliJ makes it so damned convoluted.

I do this on purpose, because it's much easier than having all in a single window. IntelliJ always gives you the option to have them in a single window.

The single window option from IntelliJ is a hack of sorts, the last I checked they have some notion about importing a module vice a project. I'm not sure why on earth you would want separate windows though when, say, you're working on multiple Spring Boot projects and a couple of library jar projects that all interact with each other. Switching back and forth from window to window seems like self cutting.