PopUpList &pt = ListObject();
	prevListSelect = pt.WhenSelect;
	pt.WhenSelect = THISBACK(listSelectCb);

// catches selections on popuplist
// to handle changes in layer's properties
void LayersDrop::listSelectCb(void)
{
	// get clicked point
	Point p = ::GetMousePos() - GetScreenRect().TopLeft();
	int col;
	if(p.x < 24)
		col = 0;
	else if(p.x < 48)
		col = 1;
	else
		col = 3;
	int iLine = ListObject().GetCursor();
	
	// if no line clicked or clicked on name, normal behaviour
	// of current layer selection
	if(IsNull(iLine) || col == 3)
	{
		prevListSelect();
		return;
	}
	
	// clicked on an item, just call given callback
	// and repost a drop on list
	Value v = GetValue(iLine);
	LayerData ld = ValueTo<LayerData>(v);
	if(col == 0)
		ld.on = !ld.on;
	else if(col == 1)
		ld.locked = !ld.locked;
	v = RawToValue(ld);
	SetValue(iLine, v);
	WhenToggle(ld.name, ld.on, ld.locked);
	PostCallback(THISBACK(Drop));
}