Collapse: count `.collapsing` elements as actives; closes #13447

Fixes #13447

Before the fix: http://jsbin.com/sayeg/1
After the fix: http://jsbin.com/sayeg/2
(Best reproducible when opening #3 and then #2.)

Tried writing a unit test but due to us disabling transitions there, it wasn't possible.

/cc @fat @cvrebert