The fix for 6580930, to cause Swing popups to overlap the Windows taskbar, has caused a regression in sandboxed applications or applets which cause a heavyweight popup to be raised:

Exception in thread "AWT-EventQueue-2" java.security.AccessControlException: access denied (java.awt.AWTPermission setWindowAlwaysOnTop)
at java.security.AccessControlContext.checkPermission(AccessControlContext.java:323)
at java.security.AccessController.checkPermission(AccessController.java:546)
at java.lang.SecurityManager.checkPermission(SecurityManager.java:532)
at java.awt.Window.setAlwaysOnTop(Window.java:2014)
at javax.swing.Popup$HeavyWeightWindow.<init>(Popup.java:215)
at javax.swing.Popup.createComponent(Popup.java:192)
at javax.swing.Popup.reset(Popup.java:130)
at javax.swing.PopupFactory$HeavyWeightPopup.getHeavyWeightPopup(PopupFactory.java:328)
at javax.swing.PopupFactory.getHeavyWeightPopup(PopupFactory.java:257)
at javax.swing.PopupFactory.getPopup(PopupFactory.java:218)
at javax.swing.PopupFactory.getPopup(PopupFactory.java:163)
at javax.swing.plaf.PopupMenuUI.getPopup(PopupMenuUI.java:44)
at javax.swing.JPopupMenu.getPopup(JPopupMenu.java:784)
at javax.swing.JPopupMenu.setVisible(JPopupMenu.java:737)
at javax.swing.JPopupMenu.show(JPopupMenu.java:910)
at com.altio.examples.applets.PopupTest$3.actionPerformed(PopupTest.java:60)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6099)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
at java.awt.Component.processEvent(Component.java:5864)
at java.awt.Container.processEvent(Container.java:2058)
at java.awt.Component.dispatchEventImpl(Component.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2116)
at java.awt.Component.dispatchEvent(Component.java:4296)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
at java.awt.Container.dispatchEventImpl(Container.java:2102)
at java.awt.Component.dispatchEvent(Component.java:4296)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:284)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

The attached test case from a customer illustrates the problem. This problem was reported on the 6uN early access forum:
  http://forums.java.net/jive/thread.jspa?threadID=38012&tstart=0