Investigate grid behavior when including iframes in cells

[coopy]

From @philholden, talking about a grid system he's working on:

We use iframes and these reload if an iframe is reparented or if a sibling before an iframe is removed from the DOM.

I am getting round this by drawing panels absolute, constraining the width measuring the height and then transitioning to the correct column and order. If an element gets removed then we need to insert a hidden placeholder div at this index in the panels array. The next panel added should reuse any placeholders in the panel array in preference to pushing on a new item.