Changes

% these values are good for small text boxesBodyFontSize should be the current font size of/in the framemw := BodyFontSize/3;ox := -0 .5 ; % offset xoy := -0 .5 ; % offset ybx := 10 1.5mw ; % bleed x(height of the shadow)by := 0 1.5mw ; % bleed y(width of the shadow)rx := 5 3mw ; % max. corner radius xry := 5 2mw ; % max. corner radius ysteps := 10 ; % number of shadow layers, 10 is a good valuehue := 0.015 ; % 0.02 is a good valueycorr := 1mw ; % difference between overlay height and shadow height

for dx step = 0 1 upto 10steps:   part := (step-1)/steps; xstep := bx * part ; % current part of bleed ystep := by * part ; crx := (rx + rx*part)/2; % use more iterations for bigger boxescurrent radius dy cry := dx (ry + ry*part)/2;  % points of the rounded rectangle xa := dx -xstep + ox ; xb := - bx/2xstep + ox + crx; xb xc := xstep + ox - crx + \overlaywidth - dx ; xd := xstep + ox + bx/2\overlaywidth; ya := dy -ystep + ycorr + oy ; yb := - by/2ystep + ycorr + oy + cry; yb yc := ystep - ycorr + oy - cry + \overlayheight ; yd := ystep - dy ycorr + oy + by/2\overlayheight;  fill (xb, ya)--- (xc, ya)... (xd, yb)--- (xd, yc)... (xc, yd)--- (xb, yd)... (xa, yc)--- (xa, yb)...cycle withcolor transparent(1, hue, black) ;