gnome_canvas_get_item_at() and transparency

[9kGemusen <9kGemusen myrealbox com>]

I would like to have one transparent rectangle ("fill_color" is NULL)
cover a set of non-transparent items. This would give me a way to have
the set of non-transparent objects act logically as one.

I run into problems, though, when I call gnome_canvas_get_item_at(),
expecting to get the transparent layer. It seems to ignore transparent
items. Are transparent items implicitly considered to be hidden? Is
there a way to recognize a position on a transparent (but not explicitly
hidden) layer? Or a better way to implement this?

Many thanks,
Gemusen