feathering from 'off screen' opacity mask
When creating a transition using an opacity mask (i.e. seamless wipe) or using a mask on any effect for that matter, the mask only takes effect once its edges hit the extremities of the frame. This creates a jarring beginning and/or end to the key-framed motion of the mask when the mask is feathered.
The mask's feathered edges should effect the frame even if the mask itself is off frame.