I have used jQuery UI Datepicker several times before but in one of my recent implementation, I started seeing this weird behavior. At the very bottom of my page there was a thin rectangular box. See the image below:
When I selected that element it turned out to be date-picker. That div disappears when the DatePicker is accessed at least once.
Looked for the problem and seems it is bug and nobody has tried to resolve it so far. But there is a workaround and that is to explicitly hide it via css.
Workaround:
#ui-datepicker-div { display: none; }
Resource: