<div id="ui-datepicker-div" class="ui-datepicker ui-widget ui-widget-content ui-helper-clearfix ui-corner-all"></div>
The styling on this causes it to be visible on the bottom of the page as a thin box with rounded borders. It disappears once I click the date control, but it's always there when the page first comes up.
I modified the following line in wwDatePicker.OnPreRender in webcontrolsjquery.prg:
lcStartScript = [jQuery('#] + this.UniqueId + [').datepicker(] + lcOption + [).attachDatepickerInputKeys();] + CRLF ; + [$('#ui-datepicker-div').hide();] + CRLF
This seemed to fix the problem without negative side effects.
--stein