Bug 148357 [Field Assist] - API - provide access for closing the proposal popup
I think that the field assist API needs more hooks to be customizable. For example : 1) There is no way to manually close/open the popup. Most of the times, this is the easier thing for a user to ask to implement his custom behaviour. 2) There is no way to intercept keystrokes/events directed to the field. 3) There is no way to control focus.
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/ContentProposalAdapter.java
*         string containing all the characters typed since the popup has
	 *         been open. <code>FILTER_CHARACTER</code> specifies the content
	 *         of the popup will be filtered by the most recently typed
	 *         character. The default is <code>FILTER_NONE</code>.
	 */
	public int getFilterStyle() {
		return filterStyle;
	}
	/**
	 * Set the integer style that indicates how keystrokes affect the content of
	 * the proposal popup while it is open.  Popup-based filtering is useful for
	 * narrowing and navigating the list of proposals provided once the popup is 
	 * open.  Filtering of the proposals will occur even when the control content
	 * is not affected by user typing.  Note that automatic filtering is not used to
	 * achieve content-sensitive filtering such as auto-completion. Filtering that is
	 * sensitive to changes in the control content should be performed by the
	 * supplied {@link IContentProposalProvider}.
	 * 
	 * @param filterStyle
	 *            a constant indicating how keystrokes received in the proposal 
	 *            popup affect filtering of the proposals shown.
	 *            <code>FILTER_NONE</code> specifies that no automatic filtering 
	 *            of the content proposal list will occur as keys are typed in the
	 *            popup.
	 *            <code>FILTER_CUMULATIVE</code> specifies that the content of
	 *            the popup will be filtered by a string containing all the
	 *            characters typed since the popup has been open.
	 *            <code>FILTER_CHARACTER</code> specifies that the content of the
	 *            popup will be filtered by the most recently typed character.
	 */