QStyle::CCScrollBar display empty rect when QStyle::SC_ScrollBarSlider is off

I'm currently working on gtk-qt-engine: http://gitorious.org/gtk-qt-engine/gtk-qt-engine

I'm trying to fix scrollbars but here what's happening:

Gtk draw scrollbar groove and then call another function to draw slider in this groove...

So, with Qt, i need to:

• Draw groove without slider
• Draw slider inside groove.

First function (drawing groove) fail:

option.subControls &= ~QStyle::SC_ScrollBarSlider;
m_qtStyle->drawComplexControl(QStyle::CC_ScrollBar, &option, &p, m_dummyScrollBar);

This draw me a scrollbar groove without slider... But it draw an empty rectangle in groove (at slider place)...

Exemple in attachement...