Window opening glitch on ubuntu 18.04

[Thiago Milczarek Sayão]

I have migrated our development environment from java-10 javafx to
java-11 and openjfx-11.

One thing I have noticed is that windows opens on the top left corner
of the screen and than moves to a center location. The weird thing is
that we can see the window moving. It appears to be a glitch.

This also happened on java-10 with the bundled javafx when setting it
to use gtk-3 instead of gtk-2.

So it appears to be a gtk-3 bug/glitch.

We use x.org (not wayland).