[QML] `ButtonLayout` does not work for `IconButton`

Bug Reports
#1

REPRODUCIBILITY (% or how often): 100%
BUILD ID = OS VERSION (Settings > About product): 3.4.0.24
HARDWARE (Jolla1, Tablet, XA2,…): XA2 Plus
UI LANGUAGE: QML
REGRESSION: (compared to previous public release: Yes, No, ?): ?

DESCRIPTION:

I tried to tart up a set of buttons used in my app and discovered IconButton in the Silica Reference documentation: https://sailfishos.org/develop/docs/silica/qml-sailfishsilica-sailfish-silica-iconbutton.html

Which looked like, and should be, a drop-in replacement for Button. However, while my two buttons inside a ButtonLayout were nicely in a row below my other widgets the icon buttons ended up a the top-left of my screen.

PRECONDITIONS:

This code:

Page {
    SilicaFlickable {
        anchors.fill: parent
        contentWidth: parent.width
        contentHeight: column.height

        VerticalScrollDecorator {}

        Column {
            id: column
            spacing: Theme.paddingLarge
            width: parent.width

            ButtonLayout {
                Button { text: "<" }
                Button { text: ">" }
            }
        }
    }
}

STEPS TO REPRODUCE:

Replace the two Buttons in the code above with:

IconButton { icon.source: "image://theme/icon-m-back" }
IconButton { icon.source: "image://theme/icon-m-forward" }

EXPECTED RESULT:

The icon buttons in the same place on the screen as the buttons.

ACTUAL RESULT:

The icon buttons end up in the top-left of the screen.