This has been bothering me for some time, finally got time to ask here.
Basically, the problem is this:
Notice the seam of the left image?
This happened when I rendered the icon as GtkImage over the blue rectangle (which is custom-drawn).
The blue rectangle is supposed to stretch lower than the lower edge.
Indeed, when the blue rectangle was rendered later than the icon, the blue rectangle covers the lower edge of the icon’s inner rectangle.
If you look closely, you could also notice that the small wedges of the icon is causing “shade” over the blue rectangle as well.
Do you know of any possible cause making the seam? It seems almost like the icon rendering leaves a few black pixels with nonzero alpha.