I think you may be right. Either that or if you choose to continue browsing without confirming the checkout, the book goes back into circulation (confirming the grace period has been eliminated in the new design). I'll have to test it with my next checkout.
So, I tested this tonight by checking out a book and leaving it on my 'Bookshelf' for exactly one hour while I browsed other titles. I then clicked on the 'Remove' button and the book disappeared (I purposely chose a book with multiple copies available so that it would still be available to other patrons).
I then checked out another title to test FizzyWater's lockout theory (it's happened to me in the past) and had no problem.
From this, I guess we can conclude that although the 'Add to Cart' feature has been done away with, there's no time limit placed on Bookshelf books that are not yet downloaded. And although it's convenient, I can see the potential for problems down the road.
There appears to be a major glitch with the book covers at BPL, though. They now cover my entire
screen and I'm on a 17" laptop!