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 :
Ubuntu18.04.3 TLS (libgtk-3-0:amd64 3.22.30-1ubuntu4)

A DESCRIPTION OF THE PROBLEM :
With Ubuntu18.04 and GTK Look and Feel, JTabbedPane doesn't show its border.
Ubuntu 16.04, nor RHEL8 don't have the problem.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
$ java -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel -jar SwingSet2.jar
Check the tabs.

EXPECTED VERSUS ACTUAL BEHAVIOR :
Expected:
Tab border should be displayed.
Actual:
No Tab border.

REPRODUCIBILITY :
This bug can be reproduced always.