This happens quite often: I start to compose a post, one that takes me a while to do, and then when I post it I find I have to log in again.
It just did that with my Cat of the Day post.
Is there a time limit to how long a user is apparently not actively participating, and if nothing shows up for a few minutes, you're automatically logged out?
No, there's no time limit for that, but your browser may be set to "go to sleep" ....
Bookmarks