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Has happened to me a couple of times recently while using code completion. Current dev build, Mustang, Ocean L&F, Linux / Gnome.
Created attachment 23356 [details] Stack trace
synchronization of contentComponent is missing. Should be fixed into NB 4.2
fixed in [maintrunk] /cvs/editor/completion/src/org/netbeans/modules/editor/completion/CompletionLayoutPopup.java,v <-- CompletionLayoutPopup.java new revision: 1.4; previous revision: 1.3