Opening sidebar shifts page on Windows

- Any browser on Windows

- Load any template in fullscreen mode (https://ampstart.com/#templates)
- Observe that the page scrollbar exists
- Open sidenav
- Observe the scrollbar is now hidden and therefore the page widens slightly.

Ideally the page shouldn't shift when the sidepanel is opened / closed.

This doesn't happen on Mac OS because the scrollbar is overlaid on top of the page.

Start by loading any AMP Start template in fullscreen mode on a Windows browser and reproduce the scrollbar change when opening and closing the sidenav. The issue names no implementation files or tests; done means the page width remains stable while the sidepanel changes visibility.

Written by the indexing model from the issue text.