cursor artifact while grabbing graphics item

[totem]

Hi!

I make an application using QGraphicsItems (Qt 4.5.3), which basically allows the user to draw polygons in a scene/view


Now, when I grab a vertex, a graphic artifact appears under my cursor (see attached picture). I suspect this is due to either the vertex bounding rect (which is computed in order to be zoom-independant), or to the cursor itself; but i can't figure out what is happening.

If anyone has a few minutes to spend on my case, the project can be found here :
project home page on google code

A Windows executable can be found here :
Windows executable download


The artifact appears when i use the [Add Vertex] tool, not when i simply move a vertex with the [Select] tool :S

I'm sorry if i'm not clear enough, i could try to be more precise if you have questions

Thank you all