Change SplitView orientation

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.15

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("SplitView jopa")

    SplitView {
        id: root
        anchors.fill: parent

        Rectangle {
            color: "red"
            SplitView.preferredWidth: root.orientation === Qt.Horizontal
                                      ? root.width / root.count : root.width
            SplitView.preferredHeight: root.orientation === Qt.Horizontal
                                       ? root.height : root.height / root.count
        }

        Rectangle {
            color: "blue"
            SplitView.preferredWidth: root.orientation === Qt.Horizontal
                                      ? root.width / root.count : root.width
            SplitView.preferredHeight: root.orientation === Qt.Horizontal
                                       ? root.height : root.height / root.count
        }
    }

    Button {
        text: "ROTATE ME"
        onClicked: root.orientation = (root.orientation === Qt.Horizontal ? Qt.Vertical : Qt.Horizontal)
    }
}