Section Editor Width Problem

I don't know with what WP version 5+ this started happening but the editor in sections appears narrow like this now.

I can't drag the bottom corner out to the right to make it wider, only down (vertically). Trying to edit sections in such a narrow column is a real hassle. The Classic Editor plugin was installed previously and I didn't have this width issue until recently. Someone in the FB group suggested I use Disable Gutenberg instead, but that didn't do anything to resolve the problem, same result. So how can I get the editor to fill the width of the section while keeping Gutenberg disabled?