After JDK-8185877 changes, old app window is not reactivatable anymore. It displays the following message:

Another instance is running PID: 17586
terminate called after throwing an instance of 'std::runtime_error'
  what(): Failed to load function "XOpenDisplay33" from "libX11.so" library
Aborted (core dumped)

Reproduced on Ubuntu 14.04 and Oracle Linux 7.1.
At least this should work on Ubuntu and Oracle Linux (for other cases we need to improve the message).