It's a basic site where the client has updated to 5.1.1 and installed Classic Editor because they can't figure out Gutenberg.

When editing a page in the visual tab each time they click within the box there is an empty line of whitespace added to the bottom of the dialog box they are trying to type into. It's hard to explain (and been even harder to search for a solution).

The extra lines aren't added to the page itself and editing in the text tab doesn't appear to be a problem. Even making basic changes means the visual tab ends up being three or four pages long and highlighting text is almost impossible. Even using the cursor keys causes the same problem.

If we go anywhere else on the dashboard and then back to the page the whitespace is gone, but as soon as mouse buttons are pressed the whitespace starts increasing again.

I'm going to try a new Wordpress install with new pages to see if it's something to do with pages created prior to the update, but thought I'd see if anyone else had encountered this problem.