I spend a lot of time in the Theme Editor page when developing sites.
Certain aspects of the way this page is laid out really slow things down. I would suggest making the Edit Area be full height. This will allow larger monitors or rotated monitors to see more of the code at one time without having to scroll. I would also like to suggest placing the different files in a dropdown much lick the dropdown to pick what Theme or Plugin you are using. This would allow more landscape for the Editor as well as make it where no matter what the size of you screen is, you only have one scroll bar on the page.