For the past six months or so I’ve been bedeviled by an intermittent problem in WordPress that has made me insane. Until today I was unable to figure out what was causing it or how to stop it, but I believe I have the fix if you’re bedeviled too.
The problem only happened on longer posts, but not consistently so. I would be typing and editing as usual, then suddenly the cursor would jump one line higher than the editing window, meaning I could no longer see what I was typing. I could confirm that the cursor was in that location by scrolling up a line, but if I edited any text it would jump up again, making it impossible to see the line I was working on. (This happened using the text editor in both normal and distraction-free modes, and the visual editor.)
My memory is that the problem began when WP changed its default settings to accommodate mobile screens and tablets. One key feature of that change was dumbing down the interface and slaving the scroll bar and mouse wheel to the post as a whole, instead of assigning functions depending on the location of the cursor, as had been the case.
If you are having this problem, click on SCREEN OPTIONS at the top of the WP screen while editing a post. A settings menu will drop down, at the bottom of which you will find a checkbox, checked, and beside it the following ironic text: Enable full-height editor and distraction-free functionality.
For real distraction-free functionality, uncheck that checkbox.
Not only should that solve the jumping problem, but if you work on a desktop it will give you back scroll-wheel control of the editing window when the cursor is in that space, instead of defaulting to scroll the whole page. (The scroll wheel will still move the whole page when you move the cursor outside the editing window.)
— Mark Barrett