Title: Slider-Handle Missing On Sliders Using GTK+ Look-And-Feel

Detailed Description:

When using the GTK+ look-and-feel, on Ubuntu 18.04, slider-controls don't have a visible 'handle' you normally click-on and drag with a mouse. If you click and drag the place where it ought to be (under the value number), it works as if it were there. It just isn't visible.

Cause Of Problem:

This is a bug in the latest version of Java. If you use Java 8, you won't have the problem.

Work-Around:

You can click and drag the place where the 'handle' ought to be, and it will work fine (you just can't see the 'handle'). You could also use some other look-and-feel (such as Nimbus).

Fix Available (in version):

The problem has been reported to the Open Java team, but no fix is yet available.