Have been speaking with @RohanM about this issue today.
His opinion is that this is current expected behaviour (unfortunately), as there was no design solution put in place for when the list of products displayed is longer than the max. height of the cart pop-over. The user can still proceed to checkout, or view their cart via the top buttons in the pop-over, however there is currently no scroll bar or elegant way of dealing with a pop-over that is longer than the available screen height.
@Kirsten keen to hear if this is your understanding? If this is expected behaviour then it’s a fairly poor user experience (which could cause the user to bounce), and one that we should add as an improvement in the backlog.