The revisioned properties of the Positioner (QQuickBasePositioner) like padding, topPadding, leftPadding, rightPadding, bottomPadding are not available if the positioner is accessed through a attached property:

import QtQuick 2.6
import QtQuick.Window 2.2

Window {
    visible: true
    Row {
        spacing: 1 // ok
        padding: 10 // ok

        anchors.fill:parent
        Rectangle {
            Positioner.spacing: 1 // ok
            Positioner.padding: 10 // Cannot assign to non-existent property "padding"
        }
    }
}

This is caused by QTBUG-40043 . Usually this is worked around by not revisioning at all for objects representing attached properties, but Positioner can be also used directly through one of its subtypes (Row, Column ...).