aMenu is being constructed to be presented to the user in response to the user's pressing on the menu widget in the title bar of a morphic window. Here, the model is given the opportunity to add any model-specific items to the menu, whose default target is the SystemWindow itself.
A bit of a hack to pass along this message to the controller or morph. (Possibly this Workspace menu item could be deleted, since it's now in the text menu.)