Missing Classes in IDEA

I use InteliJ IDEA for work when working on Java code. When the IDE doesn’t work, though, it’s incredibly distracting.

I had a problem where IDEA would not find a few auxiliary classes in my application. It would highlight them as errors in imports, and the search would find the source file, but technically not the class definition. I’d poke at the problem over a couple weeks, but I’d otherwise ignore it most the time until it seemed to be losing more and more of my classes, some of which I was actively modifying.

Re-importing the project’s Maven build didn’t fix it; re-cloning a brand new project didn’t fix it; and re-installing IDEA didn’t even work. Finally, I found IDEA’s config directories, and wiped those out to start over, and that cleared up the problem: I was able to find all my classes again.