After uploading photos and seeing the successful green check on the photos - if you click on the X in the upper right corner of the screen, you get a cancellation message. But the upload isn't actually cancelled; in fact, it is successful as you can see by looking at your library. Two screenshots below. (Maybe. There is also an issue with using the Photos button on this help form once you are already typing a message; about half the time the upload screen loads but none of the buttons work.)

