FTR, Raymond reports in a tweet (https://twitter.com/raymondh/status/1184667350953447424) that it is possible to get logged out if you ignore the warning and try to open a file in IDLE when the System Preference is set to always open documents in tabs. I was able to reproduce with macOS 10.14.6 on real hardware (I didn't see elsewhere). This is clearly a Tk issue and there has been a lot of work going on in Tk in this area since Tk 8.6.8. We have been waiting for an official 8.6.10 release to again try to update the python.org built-in Tcl/Tk after the very unsatisfactory experience with 8.6.9; until then still best to heed the warning!