EDITED: I thought that alpha blending was always on (I knew better) but what is happening I think is Qt reapplies the initializeGL state, and the only thing that this effects is drawBackground. I would just delete this post, but I imagine a notification may already be out.
Model::draw sets GL_BLEND. The final state may be left to chance. drawBackground happens before Model::draw.
EDITED: I thought that alpha blending was always on (I knew better) but what is happening I think is Qt reapplies the initializeGL state, and the only thing that this effects is drawBackground. I would just delete this post, but I imagine a notification may already be out.
Model::draw sets GL_BLEND. The final state may be left to chance. drawBackground happens before Model::draw.