我想检查一下文件何时全屏显示。我试着这样看document.fullscreen，但它不工作：

watch(document.fullscreenElement, (newValue) => {
  fullScreenActivated.value = newValue
})

它给了我这个：

[Vue warn]: Invalid watch source:  null A watch source can only be a getter/effect function, a ref, a reactive object, or an array of these types. 
  at <App> 2 runtime-core.esm-bundler.js:41

[Vue warn]: Invalid watch source:  document.fullscreenElement A watch source can only be a getter/effect function, a ref, a reactive object, or an array of these types. 
  at <App>

我知道你不能看这个元素，但我不知道如何做到这一点的另一种方式。如果有人有想法，我很乐意知道！：）