Entry editor: right-side of fields truncated

[wujastyk]

JabRef 5.0-dev--snapshot--2019-05-06--master--9b903abbf
Linux 4.15.0-48-generic amd64
Java 1.8.0_191

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

Steps to reproduce the behavior:

1. Open the editor screen
2. Select Optional fields - it has two columns of fields
3. The right-hand fields all go right to the edge of the window without closing, and the popup icons for things like urldate and DOI fetch do not display. There's no way to scroll right.