Text gets cut off in vertical split screen and full screen mode

Whenever I try to view something in either vertical split-screen mode or full screen mode, a good portion of the text just disappears partway down the document - all I get is whiteness. The writing doesn’t get deleted or anything - I just can’t see it. This doesn’t happen in horizontal split-screen or no-split mode. Is there a way I can fix this?

[attachment=0]Screenshot (65).png[/attachment][attachment=1]Screenshot (66).png[/attachment]

Update: the text is now getting cut off in all modes. This means I can’t work on it anymore.