How to set viewport margins on a QTreeView?

Lailla

I'm trying to set a margin on the top side of a QTreeView viewport.

I tried setViewportMargins but it doesn't change anything.

I also tried adjust the painting rect directly at drawRow: opt.rect.moveTop(opt.rect.y()+20);

It does added the space:

But the view glitches, when an item is hovered it disappear, and clicking also doesnt seems correct.

The same issue happens trying to modify the rect using QStyledItemDelegate.

How i could set a top margin on the viewport of QTreeView?

#define qq qDebug().noquote().nospace()

class TreeDelegate : public QStyledItemDelegate
{
public:
    TreeDelegate(QObject* parent) : QStyledItemDelegate(parent) {}

	void initStyleOption(QStyleOptionViewItem* option, const QModelIndex& index) const override
	{
        //option->rect.moveTop(option->rect.y() + 20);
		QStyledItemDelegate::initStyleOption(option, index);
	}

    void paint(QPainter* painter, const QStyleOptionViewItem& option, const QModelIndex& index) const override
    {
        QStyledItemDelegate::paint(painter, option, index);
    }
};

class MyTreeView : public QTreeView
{
public:
    MyTreeView(QWidget* parent = nullptr) : QTreeView(parent)
    {
        QStandardItemModel* model = new QStandardItemModel(this);
        setModel(model);
        setIndentation(0);
        setRootIsDecorated(false);
  
        model->setHorizontalHeaderItem(0, new QStandardItem("col1"));
		model->setHorizontalHeaderItem(1, new QStandardItem("col2"));
        for (int i =0; i< 22; i++)
        {
            QList<QStandardItem*> row;
            for (int j = 0; j < 2; j++)
            {
                row.append(new QStandardItem(QString::number(i)));
            }
            model->appendRow(row);
        }

        setViewportMargins(0, 20, 0, 0);
        setItemDelegate(new TreeDelegate(this));

        connect(new QShortcut(QKeySequence(Qt::Key_F4), this), &QShortcut::activated, [&]
        {
			setViewportMargins(0, 20, 0, 0);
        });
        connect(new QShortcut(QKeySequence(Qt::Key_F6), this), &QShortcut::activated, [&]
        {
            qq << "\nviewportMargins: " << viewportMargins();
        });
    }

    void drawRow(QPainter* painter, const QStyleOptionViewItem& option, const QModelIndex& index) const override
    {
        QStyleOptionViewItem opt = option;

		opt.rect.moveTop(opt.rect.y()+20);
        QTreeView::drawRow(painter, opt, index);
    }
};


int main(int argc, char* argv[])
{
    QApplication a(argc, argv);

    QWidget w;
    QHBoxLayout layout(&w);
    MyTreeView myTreeview;
    layout.addWidget(&myTreeview);
    w.show();
    return a.exec();
}