[Bug Report] Issue with dropdown in firefox. The dropdown shows under another blocks for 1 sec, after that it move up

### Element UI version
2.15.1

### OS/Browsers version
Win10 64bit Firefox 88.0

### Vue version
2.6.6

### Reproduction Link
https://codepen.io/slobodad/pen/YzNRPzp

### Steps to reproduce
Open the dropdown

### What is Expected?
I expect that the dropdown shows above any blocks https://prnt.sc/11sqfuv

### What is actually happening?
The dropdown shows under blocks for 1 second https://prnt.sc/11sqea5 , after that it move up https://prnt.sc/11sqfuv