Comment 5 for bug 1743768

From the "inner" shadow I'd expect an effect simulating that the filtered object was carved or pressed below the screen surface and the shadows wouldn't go over its boundaries however I'm not sure about the original intention.
For that purpose it doesn't work either.

It'd need a flood fill composited with the original shape with "out" mode, then blurred, offsetted and put atop the original shape and cut to its boundaries.
Then it'd be more realistic -though the filtered area size may still need adjustments.