Suppose that I enter in a site and, 10 seconds later, I change for another tab that contains another site. 19 minutes and 50 seconds after, I return to the original tab and close that site. Which session time is measured? 20 minutes or 10 seconds?
I think that it would be fair consider just 10 seconds, that it's total time that I've spent in the site that is being measured by Google Analytics.