I have a new Ubuntu 14.04 LTS system with a default (for gnome) Gnome 3 session and a JDK8 (b132 fcs or 8u20 b20, all the same).
Any test with realSync hangs there. First, the system does some drastic repaint (removing everything, doing something like setting display mode anew, painting everything), then it hangs with
sun.awt.SunToolkit$OperationTimedOut: 10001
at sun.awt.X11.XToolkit.syncNativeQueue(XToolkit.java:2413)
at sun.awt.SunToolkit.realSync(SunToolkit.java:1569)
at sun.awt.SunToolkit.realSync(SunToolkit.java:1501)

Note that new realSync implemented in jdk9 works OK.
I have to use jdk9 b06 or later for basebuild on this system which is wrong as we need to compare jdk9 with a released system. It is also hard to do on a farm hardware. I'm afraid a solution would be, backport of realSync to 8u, perhaps conditional or something. Gnome (if not 3) used to be a main target system for XToolkit.