Listen for transitionend events
Listen for transitionend events

In Safari, I've observed situations along the lines of:

* Changing the DOM class of an element initiates a height transition
* When the attribute change is observed, Safari still reports the body
  height as its pre-transition value
* Once `transitionend` fires, the reported height is correct

I do not observe this behavior on Chrome or Firefox, but the change
herein should not hurt in those browsers either.

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