If Canvas render target is Canvas.Image, it doesn't render until requestPaint() is called

import QtQuick 2.0

Canvas {
    id: canvas
    width: 300
    height: 300

   //renderTarget: Canvas.Image    // if this is set, canvas is blank until we press Return to call requestPaint()

   focus: true
   Keys.onReturnPressed: {
       print('repainting...')
       canvas.requestPaint()
   }

    onPaint: {
        var c = canvas.getContext('2d')
        c.fillRect(0, 0, 100, 100)
    }
}