The tab won't go away, but it will trigger a fresh http request (and thus any input in a form will disappear), I believe the behavior is the same on iOS for iPhone.
Oftentimes I’ve been underground and I’d go back into Safari to read a page I already had open, but it triggers a new request with no internet connection and then blows up.
I would hope that at least the last open tab would be held in memory or otherwise heavily cached when the signal degraded enough to prevent a reload.
Though, admittedly, I never got it to work right at actually using the offline-snapshot versions of pages when I was, er, offline. Even though it clearly had them taking up space on the disk.