OnClicking the dropdown button ,it expands and when I scroll the whole web page , dropdown button moves with the scrollbar but the expanded part remains fixed?

OnClicking the dropdown button ,it expands and when I scroll the whole web page , dropdown button moves with the scrollbar but the expanded part remains fixed?

Hi
 
I have a form in which there are many entries. One entry is of type dropdown .When i Click on dropdown ,it expands(many options pop up).Now as I move the whole web page using scrollbar ,dropdown button moves but the expanded part remains fixed.

Please suggest the solution so that expanded part also moves with the window scrollbar.I am attaching image for your easiness.