This happened to me in earlier versions of the IDE but long enough that I forgot to tread softly.
No project and no history enabled. Just 5 or 6 loaded source files.
While editing and deleting some lines, I decided to undo several actions within 1 file.
I may have hit undo past the active file cause the IDE jumped to another loaded file.
I then hit redo and the IDE went spinning wheel and I had to abort the process.
Upon restarting the IDE, I noticed it loaded all but the last file I was editing.
I now re-remember to not use undo/redo aggressively.
