I solved changing approach; I'm not using anymore _state member but int _currRow to track clicked button; here it is a sample code:

Qt Code: 
  1. ItemDelegate::ItemDelegate(QObject *parent) :
  3. {
  4.     _currRow = -1;
  5. }
  6.  
  7. void ItemDelegate::paint ( QPainter* painter, const QStyleOptionViewItem& option, const QModelIndex& index ) const
  8. {
  9.     if (!index.isValid())
  10.         return;
  11.  
  12.     QApplication::style()->drawPrimitive(QStyle::PE_PanelItemViewItem, &option, painter, 0);
  13.  
  14.     ...
  15.  
  17.     button.rect = buttonRect;
  18.     button.features |= QStyleOptionButton::Flat;
  19.  
  20.     if(_currRow == index.row())
  21.         button.state = QStyle::State_Sunken | QStyle::State_Enabled;
  22.     else
  23.         button.state = QStyle::State_Raised | QStyle::State_Enabled;
  24.  
  25.     QApplication::style()->drawControl(QStyle::CE_PushButton, &button, painter);
  26. }
  27.  
  28. bool ItemDelegate::editorEvent(QEvent *event, QAbstractItemModel *model, const QStyleOptionViewItem &option, const QModelIndex &index)
  29. {
  30.     Q_UNUSED(model)
  31.  
  32.     if( event->type() != QEvent::MouseButtonPress &&
  33.         event->type() != QEvent::MouseButtonRelease ) {
  34.         return true;
  35.     }
  36.  
  37.     if( event->type() == QEvent::MouseButtonPress)
  38.         _currRow = index.row();
  39.     else
  40.         _currRow = -1;
  41.  
  42.     ...
  43.  
  44.     QMouseEvent* mouseEvent = static_cast<QMouseEvent*>(event);
  45.     if( !buttonRect.contains( mouseEvent->pos()) ) {
  46.         return true;
  47.     }
  48.  
  49.     if( event->type() == QEvent::MouseButtonRelease) {
  50.         // call slot
  51.     }
  52.  
  53.     return true;
  54. }
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Thanks ChrisW67 for your time.