error customizing QML ScrollBar

Hi -

I've tried customizing ScrollBar as follows:

import QtQuick
import QtQuick.Controls
import colors.stylesheet

ScrollBar {
    policy: ScrollBar.AsNeeded
    minimumSize: 0.1
    contentItem: Rectangle {
        implicitWidth: 6
        radius: width / 2
    }
    background: Rectangle {
        implicitWidth: 6
        color: Colors.background
    }
}

And I'm having two problems:

1. the bar doesn't fully extend to the top (or bottom) of the scroll area. It appears to stop where the standard scrollbar arrows go (but aren't present in my customization).

1. at runtime, I get the following error:
   qrc:/qt-project.org/imports/QtQuick/Controls/Windows/ScrollBar.qml:33:49: Unable to assign [undefined] to int

Might be cockpit error, but I don't see what I'm doing wrong here.