Here is an idea for an improvement for the mask erosion parameters...
Human faces are not rectangular. The important features are more of a triangle or trapezoid. Having the ability to erode a mask at 'top, bottom, left and right' seems to miss some of the obvious areas of concern... the lower right and left corners.
I would suggest adding the ability to erode from the corners in addition to the edges, especially the lower right and left corners where funky things can happen a jawlines.
Doable?
As it stands now, trying to erode the jawline ends up eroding the left eyebrow as well... an undesired outcome. If the 'bottom' is eroded sufficiently to remove the jawline... there goes the mouth too. These lower two corners as targets for erosion would be great.