Tumbler resets currentIndex after modelChanged was emitted

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Controls 2.2

Window {
  visible: true
  width: 640
  height: 480
  title: qsTr("Hello World")

  property int setting: 4

  Row {
    Button {
      text: "click"
      onClicked: setting++
    }

  Tumbler {
    model: {
      console.log("updating model to:", setting);
      return setting;
    }
    onModelChanged: {
      var newIdx = model -2;
      console.log("updating currentIndex to", newIdx, model);
      currentIndex = newIdx;
      console.log("currentIndex was updated to:", currentIndex);
    }
    onCurrentIndexChanged: console.log("currentIndex changed to:", currentIndex)
  }
  }
}

qml: updating model to: 4
qml: updating currentIndex to 2 4
qml: currentIndex was updated to: -1
qml: currentIndex changed to: 2
qml: currentIndex changed to: 0
qml: currentIndex changed to: 2
qml: updating model to: 5
qml: updating currentIndex to 3 5
qml: currentIndex changed to: 3
qml: currentIndex was updated to: 3
qml: currentIndex changed to: 0
qml: updating model to: 6
qml: updating currentIndex to 4 6
qml: currentIndex changed to: 4
qml: currentIndex was updated to: 4
qml: currentIndex changed to: 0