RHI Metal: transparent toplevel windows don't work

I've just tried using the new RHI backend with a non-trivial application on macOS Metal. Very impressive!

However It looks like transparent toplevel windows are not yet supported. This also used to be a problem in QtQuick 2 early days (QTBUG-61909).

The following example uses a transparent window background with QSG_RHI=0 but a black background with QSG_RHI=1.

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
  visible: true
  width: 640
  height: 480
  title: qsTr("Hello World")
  color: "transparent"
  flags: Qt.Window | Qt.FramelessWindowHint

  Rectangle {
    height: 100
    width: 100
    anchors.centerIn: parent
    color: "red"
  }
}

Setting QQuickWindow::setDefaultAlphaBuffer(true) early in main.cpp does not make any difference.