i should clarify re OSX/iOS safari - recent/current versions do very weird stuff including forced refreshing of pages (mainly with tabs) and i am indeed noticing random logouts, occasionally. i dont think i have lost content to one of those yet.

but OP was clear that some of the logouts were due to inactivity, and i can still recover the content in those situations 99% of the time.

and none of that is leaving site/closing browser etc, but its possible that cookies are still involved, if browser suddenly does something to them.