Meta tag box has lower z index than footer, so appears below when question box is expanded

Question

Here I have my question box expanded so it pushes down the footer. I then click the tags box, and it's below the footer:

Here is an animation:

No other site does this - in all others the suggestions box appears over the footer (Meta.SE pictured here):

It also works correctly on Workplace.SE, the site with the most (?) similar css design to here.