Rectangle {
        id: mainRect
        anchors.centerIn: parent

        implicitWidth: 200
        implicitHeight: 25

        clip: true
        color: "gray"

        Rectangle {
            id: movingRect
            height: mainRect.height
            width: mainRect.width / 4
            color: "black"

            PropertyAnimation {
                target: movingRect
                property: "x"
                from: -movingRect.width
                to: movingRect.width * 4
                duration: 2000
                running: true
                loops: Animation.Infinite

                onFromChanged: console.warn("!!! from:", from)
                onToChanged: console.warn("!!! to:", to)
            }
        }

        width: 400
    }

// ...
//        width: 400
        Component.onCompleted: {
            Qt.callLater(() => { mainRect.width = 400; });
        }
// ...