A deadlock could occur while dragging. The Appkit thread makes a call to Java on dragMouseMoved event which calls some SwingUtilities methods which get clocked on an AWTTreeLock. EventDispatchThread sometimes tries to synchronously perform a selector on the Appkit thread and a deadlock occurs.
The simplest found way to reliably reproduce this issue is to start netbeans on JDK8 and try to drag a tab. After some time the netbeans would hang.