Thank you, Paul. Appreciate your the pen.
Unfortunately, it is not quite what I was after. I do not want to stretch the children. Just stretch/transition the parent height to match the total of the children's. And after the parent transition to apply a transition on the new children.
But the children can have different heights depending on the resolution, since they could be text.
Not quite sure how to deal with those display:none to display:block? Since so far this is the best way to get offsetHeight and estimate the total height of the children.