I notice I get this if I am away for a few hours. I get a login box. I IGNORE THE BOX AND WAIT. After a few seconds it signs in, with no typing from me, and I can continue. I'm not saying this is a good thing!
That happend to me once, also.
You log in with your AdobeID on Adobe's own server, but the forum is running on a system owned by another company. To authenticate between the two relies on a set of browser cookies, but the "remember me" cookie set on Adobe's login page is ignored by the forum software. This means that all the subsequent cookies created by the forum software expire when your browser session closes, plus there is also an idle timeout feature. There is no plan to "fix" the way this works; Adobe certainly cannot let the company running the forum have access to the AdobeID database.
You sometimes get logged out of the forums, but clicking the "Log In" button instantly reconnects you without asking for a password. That happens when the "remember me" cookie on Adobe's server is still active, so Adobe's login form instantly sends you back with a new session cookie.
That explains a lot, but all this is a very confusing situation.
Anyhow, today i logged in automatically, though i entered my pasword yesterday.
So the browser session lasts for how long?
A "session" normally lasts until you close your browser window (not just the tab), but there are lots of privacy add-ons that can influence it.
Most browsers let you look at the cookies for the currently-open page; there will be a mountain of them doing marketing tracking but the critical ones for keeping your forum login alive are the ones set by the community subdomain, such as LiSESSIONID. The cookies with a host entry at the .adobe.com top-level domain are from the AdobeID form, so if you chose "stay signed in" those should have an expiry long in the future.
So I have an email re a post Dave Merchant has added to this thread, the link brings me here. Only problem is, Dave's post isn't here at the time of looking.