There is one more place when we unsafely cast MouseEvent.getSource() to Component
and get ClassCastException is the source of the event is TrayIcon
(TrayIcon doesn't extend awt.Component)

Test to reproduce is here:
https://swinghelper.dev.java.net/source/browse/swinghelper/src/java/org/jdesktop/swinghelper/tray/JXTrayIcon.java?view=markup

For some reason it is reproduced only on Linux machines

Run this demo and click the red icon in the tray area

You'll have -

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: JXTrayIcon1 cannot be cast to java.awt.Component
        at javax.swing.MenuSelectionManager.processMouseEvent(MenuSelectionManager.java:204)
        at javax.swing.plaf.basic.BasicPopupMenuUI$MouseGrabber.eventDispatched(BasicPopupMenuUI.java:817)
        at java.awt.Toolkit$SelectiveAWTEventListener.eventDispatched(Toolkit.java:2353)