[bug report] customize the theme, and the drawer component pops up and flashes.

### Element UI version
2.15.3

### OS/Browsers version
window 10/Google Chrome版本 91.0.4472.114（正式版本） （64 位）

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/csk001/pen/KKmzgme

### Steps to reproduce
codepen中无法自定义主题，可以访问链接http://117.78.27.224:8006/theme
通过引入scss的方式切换主题。
/* 改变 icon 字体路径变量，必需 */
$--font-path: '~element-ui/lib/theme-chalk/fonts';
@import "~element-ui/packages/theme-chalk/src/index";

访问重现链接，点击第一行按钮中的【默认按钮】弹出抽屉，不正常弹出。使用默认主题可以正常弹出

### What is Expected?
切换主题，drawer能正常弹出

### What is actually happening?
弹出drawer时，组件会先弹出正常宽度，然后宽度变小，再显示正常宽度

Start with the provided CodePen and the http://117.78.27.224:8006/theme reproduction, using the listed Element UI 2.15.3 and Vue 2.6.10 versions. Compare the Drawer opened with the default theme against the SCSS-customized theme imported from packages/theme-chalk/src/index. Done means the Drawer opens at its normal width without first flashing at a smaller width.

Written by the indexing model from the issue text.