gtk:snapshot-append-inset-shadow (snapshot outline color dx dy spread radius)

snapshot -- a gtk:snapshot object
outline -- a gsk:rounded-rect instance for the outline of the border
color -- a gdk:rgba instance for the color of the shadow
dx -- a number coerced to a single float for the horizontal offset of the shadow
dy -- a number coerced to a single float for the vertical offset of the shadow
spread -- a number coerced to a single float representing how far the shadow spreads towards the inside
radius -- a number coerced to a single float representing how much blur to apply to the shadow

Appends an inset shadow into the box given by outline.