In the .deb version of Greenfoot (on ArchLinux, which takes the deb package to create an AUR package) as well as in 18.04 Ubuntu a grey rectangle appears in the UI. One / several separate window(s) / program(s) in Ubuntu's left shortcut bar as well as Arch Linux's task bar (XFCE) is/are generated. In Arch Linux they show up as "java lang Thread".
This behaviour prevents you from typing any longer and you have to click into the UI to go on typing. You can then manually close the windows (which will accumulate the more often you use the autocomplete function).
This behaviour has been obvious for a long time throughout different Java versions, especially Java 8.
The official Oracle Java 8 versions as well as the openjdk versions are equally affected.
Unfortunately, I cannot add a screenshot here.
I hope you can reproduce the issue.
I also hope there will be a new version of Greenfoot and the project is not dead.