If a theme does not opt into site-wide padding through theme.json, and the user does not specify site padding through Global Styles, the post/page editor currently provides no left/right margins to prevent text from bumping up against the edges of the screen:

This is sort of an issue for block themes, since it effectively requires them to supply either a site-wide padding value, or some custom CSS. More importantly though, it's a regression in the editor experience for users of classic themes that don't manually provide these margins.

To reproduce:

1. Activate an older theme, like Twenty Eleven.
2. Open the post editor, and either switch to one of the device previews (tablet/mobile), or make your browser window smaller.
3. Note that the text bumps against the edges fo the screen.

Gutenberg version: Version 11.8.0-rc.2
Related: WordPress/twentytwentytwo#144

cc @youknowriad since this may be related to removing the site-wide margin.