As a collaborator on Sponge's trial, I think I can give some more information as it is happening on my end too (For the record, her is my user agent string: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36).
First off, it appears to be the same issue reported here: viewtopic.php?f=14&t=12176
The solution presented in the other thread notes that zooming out a bit helped, and it did; however, it is inconsistent. Once the popup is exited and re-entered, the scroll pane proceeds to have the same issue as before. I've attempted this process on some my trials to see if if it was exclusive to the Sponge's trial or not. I found that this issue was consistent, meaning it is either a problem with the browser or the editor itself. I will continue to experiment with it until we can find a reasonable solution.
Edit: I can no longer reproduce the error. I am on a different cpu at my college, but the setup should be virtually the same.