[cssom] Sticky position percentage values for inset properties. #3115
Description
Consider the test-case:
<!doctype html>
<div id="log"></div>
<div style="height: 500px; background: purple; overflow: hidden;">
<div style="height: 400px; background: yellow;">
<div id="target" style="height: 100px; position: -webkit-sticky; position: sticky; left: 0; top: 50%; background: blue;">
</div>
</div>
</div>
<script>
log.innerHTML = getComputedStyle(target).top;
</script>Every browser resolves the percentage during layout relative to the overflow: hidden container (that is, the closest scrollable box), so #target is positioned 250px from the top.
However, every browser except Gecko resolves the percentage in getComputedStyle against the containing block (the yellow box), returning 200px instead.
I think I should file bugs against the other browsers and clarify the spec because those percentages make no sense, but I'm not sure if I'm missing something...