Consider this test case:

import QtQuick 2.5
import QtQuick.Controls 2.5 as QQC2

Item {
    implicitWidth: 200
    implicitHeight: 200

    QQC2.SpinBox {
        anchors.centerIn: parent
        width: 50
        editable: true
        from: 0
        to: 9999999
    }
}


Enter "9999999" in the box and the text will overflow. It should be elided or clipped, at least, to mirror the behavior of a text field.