title: DropDownList doesn't postback on SelectedIndexChanged
body: <p>I'm writing an ASP.Net webform with some DropDownList controls on it. Then user changes selected item in one of dropdowns, ASP.Net doesn't seem to handle SelectedIndexChanged event until form is submitted with a 'Submit' button click.
  How do I make my dropdowns handle SelectedIndexChanged instantly?</p>
  
  <p>P.S. It's a classic question I have answered too many times, but it seems no one asked it before on stackoverflow.</p>