FULL PRODUCT VERSION :
openjdk version "14-ea" 2020-03-17
OpenJDK Runtime Environment (build 14-ea+17-721)
OpenJDK 64-Bit Server VM (build 14-ea+17-721, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Ubuntu 18.04.3 LTS
Ubuntu 16.04.6 LTS

A DESCRIPTION OF THE PROBLEM :
Left side border of JComboBox was truncated with GTK Look and Feel.
This problem occurs with Ubuntu 16.04 and 18.04.
This problem didn't occur with RHEL7, 8 and SLES12.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
$ java -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel -jar SwingSet2.jar
Move to JComboBox demo.

EXPECTED VERSUS ACTUAL BEHAVIOR :
Expected:
Full width of JComboBox should be displayed.
Actual:
Left side of each JComboBox was truncated.

REPRODUCIBILITY :
This bug can be reproduced always.