Thanks for the report and apologies for the confusion.
The Cancel button is intended as an alternative to the Send button, and shouldn't be accessible to you once you've clicked Send. The bug on our end is that we're not disabling the Cancel immediately after you click Send. We'll get that fixed.
Ah, I understand. In fact the whole screen is still clickable, including the list of files that are already being sent but you can still add things to the list - confusing.
I do actually want a cancel button. Maybe I should say, "need," a cancel button? Especially on very big files, you have time to think and realize you've forgotten something or made an earth-shattering mistake. Right now I have to exit the browser, which does effectively stop the send.
Yes, the entire screen should be deactivated. There should be a gray mask that goes over it until the upload is complete. It did work that way in the past; we're not sure when the behavior changed.
I will pass along your request for a cancel option to the product team.