ColorDialog ShowAlphaChannel option doesn't work on GTK

        ColorDialog {
            id: borderColorDialog
            selectedColor: "sienna"
            options: ColorDialog.ShowAlphaChannel
            // workaround: put a slider somewhere else in the UI
            property color alphaAdjusted: Qt.rgba(selectedColor.r,
                                                  selectedColor.g,
                                                  selectedColor.b,
                                                  borderAlphaSlider.value)
        }