Transitioning from month to month when the months have different weeks are slightly off.

To reproduce, use the arrows to transition from December 2017 to November 2017. Observe the cell spacing during the animation. The cells should slide seamlessly into view, but there's a noticeable gap. Custom cells with transparent backgrounds might see this as overlap as well.

When the months before and after the transition have different numbers of weeks, this becomes an issue.

I think this is because we're using the contentSize of the collection view prior to transitioning as the offset for the new cells. This is probably wrong (or my understanding of how I'm doing the measurements is wrong.)