can't use getComputedStyle when the component is mounted

[liuyang0826]

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Describe the bug

When the component is mounted, getComputedStyle will get default style instead of the style in CSS

System Info

• required vite version: 0.15.0
• required Operating System: window10 x64. chrome 80
• required Node version: 14.2.0
• Optional:
  • npm/yarn version
  • Installed vue version (from yarn.lock or package-lock.json)
  • Installed @vue/compiler-sfc version

Logs (Optional if provided reproduction)

1. Run vite or vite build with the --debug flag.
2. Provide the error log here.

css:

.content {
  line-height: 24px;
}

js:

const contentRef = ref(null);
onMounted(() => {
  let { lineHeight, height } = getComputedStyle(contentRef.value);
  console.log(height, lineHeight);  // 63px normal
  showBtn.value = parseFloat(height) / parseFloat(lineHeight) > 2;
});