I think I've narrowed it down to files that have special status bars at the top, like HTML, XML and perhaps even build.sbt files. It's super annoying. This has only been an issue since 2016.x.
Anyone else seen this? I hate the clutter of tabs and generally use shift-shift to switch between recently opened files. In this scenario though, the split window I attempt to open a file in effectively disappears and can only be brought back by 1) unsplitting, resplitting and not opening the bad file; 2) re-enabling tabs; 3) not using splits at all.
It's totally ruined how I enjoy working in IDEA.