Aye... OpenGL cannot draw concave (C shaped) polygons... that screws up the Lasso tool, when it uses backbuffer selection. Examined for little while the GLU Tesselation library, but apart from its nightmarish structure, it's even stupid (no builtin clock/counterclock). So, instead coded a DispList based function using Blender's edgefill. Works like a charm! :)