QT Double buffering does not work on the board

[ForestPoem]

The PC will operate normally , but the image output is not smooth in the board.
I checked and double-buffering support in QT itself, I understand that.
Why not apply ? Below are the specific signal occurs when the behavior of the function.

void curvePlotter::keyPressEvent(QKeyEvent *event)
{

switch (event->key()) {
case Qt::Key_Plus:
    zoomIn();
    break;
case Qt::Key_Minus:
    zoomOut();
    break;
case Qt::Key_Left:
    zoomStack[curZoom].scroll(-1, 0);
    refreshPixmap();
    break;
case Qt::Key_Right:
    zoomStack[curZoom].scroll(+1, 0);
    refreshPixmap();
    break;
case Qt::Key_Down:
    zoomStack[curZoom].scroll(0, -1);
    refreshPixmap();
    break;
case Qt::Key_Up:
    zoomStack[curZoom].scroll(0, +1);
    refreshPixmap();
    break;
default:
    QWidget::keyPressEvent(event);
}

}

void curvePlotter::refreshPixmap()
{
pixmap = QPixmap(size());
pixmap.fill(this, 0, 0);
QPainter painter(&pixmap);
painter.initFrom(this);
drawGrid(&painter);
drawCurves(&painter);
update();
}