Multiple background task popups stacked over each other

[calixtus]

JabRef version 100,0,0 (12.5.2020 22:00 pm) on Windows 10 10.0 amd64 Java14

• I have tested the latest development version from http://builds.jabref.org/master/ and the problem persists

Steps to reproduce the behavior:

1. Click on the background tasks button in the tollbar.
2. Click on it again
3. And again

You can see that the shadow of the popup is getting darker each time.