Content is not sized correctly for the viewport

# Bug report

**What is the current behavior?**

The viewport size is 424px, whereas the window size is 412px. This can be seen on mobile phone size screens when you swipe left the page moves.

**If the current behavior is a bug, please provide the steps to reproduce.**

## Mobile
- Go to https://gatsby-netlify-cms.netlify.com/
- Swipe left
- the screen moves and you get a white bar down the right hand side

## Lighthouse
- Go to https://gatsby-netlify-cms.netlify.com/
- Run Lighthouse tests
- Under PWA Optimized -> "Content is not sized correctly for the viewport"

**What is the expected behavior?**

window.innerWidth === window.outerWidth

https://developers.google.com/web/tools/lighthouse/audits/content-sized-correctly-for-viewport

**Other relevant information:**

None

Node.js version: N/A
NPM/Yarn version N/A
Operating System: N/A
Additional tools: N/A