Proper custom control sizing?

[RogueWarrior]

I've been working on a variation of the GroupBox control where the label lives outside the content rectangle (above the upper left corner) so it looks a bit like a filing cabinet folder tab. But I'm having some difficulty getting the bounding box of it to account for this label in column layouts so the spacing looks right.

T.GroupBox {
    id: control

    implicitWidth: Math.max(implicitBackgroundWidth + leftInset + rightInset,
                            contentWidth + leftPadding + rightPadding,
                            implicitLabelWidth + leftPadding + rightPadding)
    implicitHeight: Math.max(implicitBackgroundHeight + topInset + bottomInset,
                             contentHeight + topPadding + bottomPadding)

    spacing: 6
    padding: 0
    verticalPadding: 0
    topPadding: 0
    horizontalPadding: 0
    bottomPadding: 0

    label: Label {
        text: control.title
        color: control.palette.windowText
        anchors.leftMargin: 10
        anchors.bottom: parent.top
        topPadding: 0
        bottomPadding: 0
        leftPadding: 10

        background: Frame {
            z: -1
            y: -1
            width: parent.width + 10
            height: parent.height + 2
        }
    }

    background: Frame {
        anchors.top: control.top
        anchors.fill: parent
    }
}