Most every morning I sit down, start up my computer and start up all the programs I will be working with that day. I usually proceed to read my email first. Unfortunately, Intellij, like so many products, thinks it is the most important program ever and it must have my attention. So it steals focus and puts itself right in my face. However, I tend to have between 2-4 projects open in different windows. So it does this not only once but for each project.
IMO there is never a case for focus stealing. One could make a case for security software which may have a legitimate need to demand direct action. I also think such a case has never occurred in my life. I suppose any kind of alarm program could make the case. I still wouldn't agree, but I can see that some people would. However IntelliJ has no case whatsoever to steal focus. Focus should be on what I am focussed on. Just because I started the IDE does not mean I intend to work with it right now. Untill someone implements a telepathic focus algorithm programs should just start up in the background and let me continue with whatever I am doing.