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
* 
	 * @param listener
	 *            the IContentProposalListener2 to be removed as a listener.
	 *            Must 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.IContentProposalListener2
	 */
	public void removeContentProposalListener(IContentProposalListener2 listener) {
		proposalListeners2.remove(listener);
	}
	/*
	 * Add our listener to the control. Debug information to be left in until
	 * this support is stable on all platforms.
	 */
	private void addControlListener(Control control) {
		if (DEBUG) {
			System.out
					.println("ContentProposalListener#installControlListener()"); 
		}
		if (controlListener != null) {
			return;
		}
		controlListener = new Listener() {
			public void handleEvent(Event e) {
				if (!isEnabled) {
					return;
				}
				switch (e.type) {