Negative Coordinates clamped to 0
Native X11 applications cannot have negative X,Y coordinates, they are set to 0. Therefore it is not possible to move windows over the screen border.
Please see: https://bugs.openjdk.java.net/browse/JDK-8222794 and https://github.com/javafxports/openjdk-jfx/issues/448 For more details. The latter link also has some C code to demonstrate the behavior.