Out-of-flow positioned boxes usually don't affect the size of their ancestors (with the exception of any scrollbars that an OOF might cause).

However:

<p>Should there be a yellow 100px tall multicol container below, with three
  columns, with one blue square in each?</p>
<div style="columns:3; background:yellow;">
  <div style="position:relative;">
    <div style="position:absolute; width:100px; height:300px; background:blue;"></div>
  </div>
</div>

Gecko and Blink show three blue squares in a 100px tall yellow multicol container here, i.e. the OOF affects the height of the multicol container. Is this how we want things?