Layout.fillWidth/Height do nothing if the width/height are 0

import QtQuick 2.1
import QtQuick.Layouts 1.0

RowLayout {
  width: 100
  height: 100
  
  Rectangle {
    Layout.fillWidth: true
    color: 'red'
    //width: 1  // uncomment to make the red rectangle show up
    height: 10
  }
}