Oh man, this is disappointing.
I just tried it with 3 browsers (safari, firefox, and chrome) - on 2 different computers! And, the problem occurs exactly as I have described it. The popup stops occurring if I return to the store page to add additional products.
If I don't go to the checkout procedure, I can add as many products as I want, and it works just fine.
As soon as I go to the checkout procedure and select a shipping method, then if I go back to the store I don't get the pop-up even though products are still being added to the cart. The user just doesn't get any indication that it is working.
Even though it is working fine for you, do you have any recommendations for me? I can't switch back to the legacy store because of the problem with coupon restrictions that I posted before (I stopped worrying about that issue because it that works in the new store). So, I'm kind of stuck here.
After you started the checkout process and entered and address, did you use the "store" menu tab to return to the store or get back to entering additional products a different way? There must be something different about what we're doing that causes the difference in behavior. (Also, I'm on a Mac...maybe there's an issue there?)
UPDATE WITH ADDITIONAL INFORMATION: I also just tried it with a PC running Windows and IE. Exact same result. I did it on another Mac, as well. So, now I have recreated it on 4 machines using 4 browsers, and it's the same on every one.
When you are testing this, please be sure that after you get to the checkout process, you then select the store menu item from the upper right to return to the store. That's the process I've been using.
Thank you for any help you can offer.
Scott