Say we have a long list, so some items may not be seen at the beginning.
After user scrolls the screen, some more items are seen.
<ul>
<li>
<li>
<li>
--------not seen at the beginning-------
<li>
<li>
<li>
....
</ul>
What is the best practice for judging which item is seen by the user?
Maybe known as Impression, but I couldn't find anything about it.