Create a new layer above every layer that contains objects that you want hidden
when off the stage.Create a rectangle the same size as your stage and set the X
and Y coordinates to 0.

Double click on the layer properties icon (next to the layer name) and set it
to "Mask". Then move the layers you want the mask applied to underneath it.
They should appear slightly indented with an icon next to them to show that the
mask has been applied to them.

Bit of a pain but it seems to work fairly well. Just had a similar issue.