Layout shift when accessing search or scrollbar appears

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues).

### Prerequisites

- [X] I'm using the latest version of Docusaurus.
- [ ] I have tried the `npm run clear` or `yarn clear` command.
- [ ] I have tried `rm -rf node_modules yarn.lock package-lock.json` and re-installing packages.
- [ ] I have tried creating a repro with https://new.docusaurus.io.
- [ ] I have read the console error message carefully (if applicable).

### Description

When you click the search bar to open the modal there is a slight layout shift on the doc content.


### Reproducible demo

https://docusaurus.io/

### Steps to reproduce

1. Visit https://docusaurus.io/
2. Click search and see the layout shift.

### Expected behavior

No layout shift.

### Actual behavior

N/A

### Your environment

- Public source code:
- Public site URL:
- Docusaurus version used:
- Environment name and version (e.g. Chrome 89, Node.js 16.4):
- Operating system and version (e.g. Ubuntu 20.04.2 LTS):

### Self-service

- [ ] I'd be willing to fix this bug myself.