On Mac an application displaying 2 or more stage stops responding when changing z-order of its windows

To reproduce:
Load the attached project in NetBeans
Resolve properties to point to jfxrt.jar
Run it: you get two windows, one containing a Chart, the other one a Text and a Button. Bring the second window to the front, that might be enough to enter the lock. In case you may have to move one or the other window to modify z-order.