client/components/fullscreenButton.js

const FullscreenButton = scene => {
  let button = scene.add
    .image(scene.cameras.main.width - 20, 20, 'fullscreen', 0)
    .setOrigin(1, 0)
    .setInteractive()
    .setScrollFactor(0)
    .setDepth(100)
    .setAlpha(0.2)

  button.on('pointerup', () => {
    if (scene.scale.isFullscreen) {
      button.setFrame(0)
      scene.scale.stopFullscreen()
    } else {
      button.setFrame(1)
      scene.scale.startFullscreen()
    }
  })
  return button
}

export default FullscreenButton