public interface ComboBoxFiltering
 While the user is typing in a value, the ComboBoxItem.getPickListFilterCriteria() method will return the typed-in value as part of the
 criteria, so that only matching values are shown.  Matching is determined by the textMatchStyle.  Note
 that the ComboBoxItem.filterFields attribute may be used to determine which fields filtering is
 performed against for databound comboBoxItems. 
If the user explicitly shows the down-down pickList, via either clicking on the drop down icon or using the Ctrl+Arrow Down key combination, the typed-in value is ignored for filtering.
 If included in the criteria, the
 typed-in value will be included as a value for the  displayField (or for the
 valueField if
 this.displayField is unspecified). 
 Static criteria, specified via optionCriteria or pickListCriteria,
 will always be included by the default implementation (combined with the typed in value if
 appropriate). 
 For custom pick list filter criteria, we recommend  calling the  com.smartgwt.client.widgets.form.fields.ComboBoxItem.setPickListFilterCriteriaFunction method.
 If you are implementing your own pickList filter criteria, the read-only property this.filterWithValue
 can be read to determine whether the ComboBox would ordinarily ignore the typed-in value for
 filtering.  Note that in addition to cases where the user explicitly shows the pickList,
 filterWithValue will also be true during a call to  ComboBoxItem.fetchData() on a
 databound comboBox. 
 NOTE: The defaut implementation of this method will return an 
 AdvancedCriteria object if multiple ComboBoxItem.filterFields
 are specified, or if there are field collisions between any specified static optionCriteria, pickListCriteria and
 the entered value. AdvancedCriteria are not supported by all DataSource types, including the
 built-in server-side SQL dataSources in Smart GWT Pro edition (though they are supported by SQL
 dataSources in Power and Enterprise editions). 
 Client-Side Filtering
 By default,
 the ComboBoxItem will automatically use client-side filtering whenever it receives a complete
 set of results for a given search string, and then the  user types more letters (so reducing
 the results further). 
 Client-side filtering may malfunction if the server filtering
 behavior can't be  replicated client-side (for example, Google Search).  To disable client-side
 filtering so that the comboBox always contacts the server for data whenever the  use changes
 the search string, set ComboBoxItem.useClientFiltering to false. 
However, disabling client-side filtering will slow down the UI and cause more round-trips to the server, so if client-side filtering is malfunctioning but should work, try to correct the problem rather than disable the feature.
For example, if the initial search works correctly but adding more letters always causes zero matches, most likely the Records returned by the server lack values for the field(s) targeted by the filter criteria, or the field values returned by the server don't match the criteria values.
 View the returned data in the RPC tab in the Developer Console
 and enable the "ResultSet" log category in the "Results" tab to troubleshoot how the filter 
 criteria are being applied to data, and look closely at your settings for valueField and displayField.