Page/Post editor: grey border appears on either side of the editor when using a classic theme

[annezazu]

I noticed today while doing some testing on a simple site with classic themes, like Twenty Seventeen, that there's a grey border/padding on either side of the editor when writing a new post or page:

I can't replicate this in Core on Playground with GB installed + Twenty Seventeen: https://playground.wordpress.net/?plugin=gutenberg&theme=twentyseventeen It seems WordPress.com specific.

[github-actions[bot]]

OpenAI suggested the following labels for this issue:

• [Feature Group] Appearance & Themes: The issue relates to the appearance of the post/page editor in classic themes, which is affected by a visual anomaly.
• [Feature] Classic Interface: The problem is reported while using classic themes, indicating that it concerns the traditional WordPress interface.
• [Feature] Post/Page Editor: The grey border issue directly affects the post and page editor, highlighting a problem with editing functionality.