transparency.makeOpacityMask with two or more layers below the mask
object (or a layer and objects of other layers) wrapped the layers
themselves in a new top-level group, so a layer ended up inside a
group. A layer among several masked items now stands for its art: the
masked group takes the layer's visible, unlocked objects, a hidden,
locked or template layer gives none, and the layers stay where they
are. One layer under a mask object, or one layer alone, is masked as
before.