Edit screen is cut off

Just started happening after several years with no problems. When I want to edit a page, the Pages nav bar is okay at the side, but the main part of the page is cut off and only shows about two inches' worth.

There's a scroll bar at the side, so I can edit pages, but you'll understand that trying to edit a page when viewing only a two inch deep strip is, er, annoying!

Have I accidentally altered a setting or something? Help please.

See https://answers.squarespace.com/questions/284158/google-ad-code-no-others-impacts-edit-mode-five-te.html

For more information, contact Squarespace Customer Care. You can reach out to them here.