I'd like to suggest that the Microsoft Sync Pop-Up window be unlocked from allowing access to tabs or other Microsoft Edge browsing areas. I am sure that there are, or will be other users who need to access a tab or browse to another page at the same time as they are signing up and logging into Sync, to retrieve information, in-particularly the confirmation code. For example, all of my codes are received SMS, but are relayed automatically to an e-mail account, so it's not as simple as picking up my android or ios device to retrieve the confirmation code. When working at a computer, I need to be able to switch within the browser to a website to access my e-mail to retrieve the code. If I close the frozen pop up sync window then I am leaving it without having completed the sync connection and the code needed to be retrieved will no longer be valid.
This shouldn't be a difficult fix, considering its not an issue in Vivaldi or Chrome browser sync.
I'm also not sure why it would be a locked pop-up window anyway.
(Windows 8.1 version of Microsoft Edge Version 78.0.276.17 (Official build) beta (64-bit)