title: Modal popup outside browser viewport
body: <p>I have a popup on my website for some registration.
  When this popup opens on a phone, it is to high for the browser view port, and some of the content comes outside it.</p>
  
  <p><img src="https://i.stack.imgur.com/wahr6.png" alt="illustration of the problem"></p>
  
  <p>When I try to scroll down, it scroll the background. If I lock the background with <code>overflow-y: hidden</code>, nothing scrolls at all. </p>
  
  <p>The popup scrolls with the window, locked at the same place.
  I would like to not lock the position to the top of the page, and have it always appear in the top of the view port, but it should be possible to scroll to see all its contents. </p>
  
  <p>Does anyone know how to fix this?</p>