jensnorr and kozie are both right in my case, too. I used ctrl-cmd-p to cycle thru all of my projects, making sure all were set to hide open files, quit and restarted a couple of times, and the problem remained. I then closed the editor window without quitting, and there was a window open behind it displaying a project that, for whatever reason, wasn't listed in the project palette. It was showing open files. I set it to hide and closed that window, quit, restarted, cmd-n to open a window, ctrl-cmd-p for the project palette, chose a projects, and all was working again.
I hope this is enough detail to help people work around this.