We had an issue where an open Dialog got hidden by css didn't properly
unmount because the Transition never "finished". We fixed this by
checking if the node was hidden by using `getBoundingClientRect`.
Today I learned that just *reading* those values (aka call
`node.getBoundingClientRect()`) it for whatever reason completely stops
the transition. This causes the enter transitions to completely stop
working.
Instead, we move this code so that we only check the existence of the
Node when we try to transition out because this means that the Node is
definitely there, just have to check its bounding rect.
Robin Malfait
ce12406ec2