Flash MX 2004.
timeline.showLayerMasking(Animate[layer])
layer A zero-based index of a mask or masked layer to show masking during authoring. This parameter is optional.
Nothing.
Method; shows the layer masking during authoring by locking the mask and masked layers. This method uses the current layer if no layer is specified. If you use this method on a layer that is not of type Mask or Masked, Animate displays an error in the Output panel.
The following example specifies that the layer masking of the first layer should show during authoring.
fl.getDocumentDOM().getTimeline().showLayerMasking(0);