Use test from bug #4145096 and try resizing the internal frame. You get:

Exception occurred during event dispatching:
java.lang.ClassCastException: java.awt.Frame
        at
javax.swing.plaf.basic.BasicInternalFrameUI$BorderListener.mousePressed(BasicInt
ernalFrameUI.java:805)
        at java.awt.Component.processMouseEvent(Component.java:3739)
        at java.awt.Component.processEvent(Component.java:3571)
        at java.awt.Container.processEvent(Container.java:1178)
        at java.awt.Component.dispatchEventImpl(Component.java:2620)
        at java.awt.Container.dispatchEventImpl(Container.java:1227)
        at java.awt.Component.dispatchEvent(Component.java:2524)
        at
java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:2469)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:2228)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:2143)
        at java.awt.Container.dispatchEventImpl(Container.java:1214)
        at java.awt.Window.dispatchEventImpl(Window.java:912)
        at java.awt.Component.dispatchEvent(Component.java:2524)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:325)
        at
java.awt.EventDispatchThread.pumpOneEvent(EventDispatchThread.java:103)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:84)
Exception occurred during event dispatching:
java.lang.ClassCastException: java.awt.Frame
        at
javax.swing.plaf.basic.BasicInternalFrameUI$BorderListener.mouseReleased(BasicIn
ternalFrameUI.java:708)
        at java.awt.Component.processMouseEvent(Component.java:3742)
        at java.awt.Component.processEvent(Component.java:3571)
        at java.awt.Container.processEvent(Container.java:1178)
        at java.awt.Component.dispatchEventImpl(Component.java:2620)
        at java.awt.Container.dispatchEventImpl(Container.java:1227)
        at java.awt.Component.dispatchEvent(Component.java:2524)
        at
java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:2469)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:2234)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:2143)
        at java.awt.Container.dispatchEventImpl(Container.java:1214)
        at java.awt.Window.dispatchEventImpl(Window.java:912)
        at java.awt.Component.dispatchEvent(Component.java:2524)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:325)
        at
java.awt.EventDispatchThread.pumpOneEvent(EventDispatchThread.java:103)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:84)