JabRef 5.1--2020-08-30--e023aa0
Linux 5.4.0-42-generic amd64
Java 14.0.2

Steps to reproduce the behavior:

1. Resizing JabRef window works fine on opening, scaling column width with window size.
2. Have more than 2 files/tabs open.
3. Change the width of a column (e.g., Year).
4. Enlarge the JabRef window width.
5. JabRef responsively shrinks all the columns, except the rightmost (as if you were reducing window width).