WYSIWYG editing not using full window height

I created a new page, and added some bullet points. Then I noticed that all my editing was occurring in a small scrollable window at the top of the page instead of using the full page height (screenshot attached).

Saving the page and re-editing has no effect; switching the browser from full-screen to windowed mode has no effect; restarting the browser has no effect.

Somehow the page is stuck in 'edit in tiny window' mode.