Why is image duplicated in editor when monitor is in portrait mode?

I'm a web designer and I often use Photoshop with my monitor rotated 90 degrees so I can see the whole page at once. However, recently that has become impossible. See the attached image. The file appears twice and is sometimes cut off depending on zoom level, file size, etc. Even worse, only the bottom image allows interacting with tools. This happens in fullscreen or standard screen mode.