Bug 19275 editor: context menu slow on large files
F2 it's slow, but only the first time after a change. then it's fast afain you make another change - it's slow again
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/compare/JavaHistoryAction.java
}
			
	protected boolean isEnabled(ISelection selection) {
		IMember m= getEditionElement(selection);
		if (m == null)
			return false;
		IFile file= getFile(m);
		if (fModifiesFile && file != null && file.isReadOnly())
			return false;
		if (file != null && beingEdited(file))
			return getWorkingCopy(m) != null;
		return true;
	}
	
	/**
	 * Notifies this action delegate that the selection in the workbench has changed.
	 * <p>
	 * Implementers can use this opportunity to change the availability of the
	 * action or to modify other presentation properties.
	 * </p>
	 *
	 * @param action the action proxy that handles presentation portion of the action
	 * @param selection the current selection in the workbench
	 */
	public void selectionChanged(IAction action, ISelection selection) {
		fSelection= selection;
		action.setEnabled(isEnabled(selection));
	}
}