title: Is my understanding of document and viewport with regards to mouse position in javascript correct?
body: <p>Based on answers <a href="https://stackoverflow.com/questions/7089842/difference-results-using-pagey-and-clienty-mouse-positions">from a previous question</a>, both refer to mouse positions (x and y coordinates). </p>
  
  <ul>
  <li>relative to the document and  </li>
  <li>relative to the viewport.</li>
  </ul>
  
  <p>I have read through an article on <a href="http://www.quirksmode.org/mobile/viewports.html" rel="nofollow noreferrer">QuirksMode</a>, however I think I may be missing something. I have put together these two diagrams to help me in my understanding. Is my analysis correct?</p>
  
  <p><img src="https://i.stack.imgur.com/7tNsa.png" alt="enter image description here"></p>
  
  <p>Now scroll the document 250px...</p>
  
  <p><img src="https://i.stack.imgur.com/BaYiN.png" alt="enter image description here"></p>