Compile and run the attached test (2 files and a picture) on OEL 7.1 in multiscreen mode (I'm not sure if it is reproducible on a single screen: the insets bug JDK-8134669 is not).
Eventually it will push a java.awt.Window to fullscreen mode and query its location by getLocationOnScreen.
Now, Window.toString() reports it as [0,0,couple-of-big-numbers] with zeros in parent coordinate system but getLocationOnScreen() reports [0,top-inset-size] ( in my case, [0, 28] ).
Note also, that getInsets() return all zeros (see JDK-8134669).

Reproducible at least on jdk9 and 8 so that may be some JDK+Gnome 3 issue.