Bug 256651 [FieldAssist] Wrong location calculation of proposal popup shell if content control is near bottom in the screen
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/ContentProposalAdapter.java
*            not be <code>null</code>. If the listener has not already
	 *            been registered, this method has no effect.
	 * 
	 * @since 3.3
	 * @see org.eclipse.jface.fieldassist.IContentProposalListener
	 */
	public void removeContentProposalListener(IContentProposalListener listener) {
		proposalListeners.remove(listener);
	}
	/**
	 * Add the specified listener to the list of content proposal listeners that
	 * are notified when a content proposal popup is opened or closed.
	 * </p>
	 * 
	 * @param listener
	 *            the IContentProposalListener2 to be added as a listener. Must
	 *            not be <code>null</code>. If an attempt is made to register
	 *            an instance which is already registered with this instance,
	 *            this method has no effect.
	 * 
	 * @since 3.3
	 * @see org.eclipse.jface.fieldassist.IContentProposalListener2
	 */
	public void addContentProposalListener(IContentProposalListener2 listener) {
		proposalListeners2.add(listener);
	}
	/**
	 * Remove the specified listener from the list of content proposal listeners
	 * that are notified when a content proposal popup is opened or closed.
	 * </p>