"Flip image" button under Sequence/Program window
In the button editor under the Program tile/window, I wish there was a button that would easily toggle on/off a horizontal image flip. A lot of artists/graphic designers/photo editors use this function in other software because it's a lot easier to see what you are doing. You get blind to any changes when you look at the same image for an extended period of time, but when easily flipping back and forth beetween a flipped image your mind can see the changes you made much more easily. It makes for a way better creative process during color correction etc, and you save a lot of time tweeking. If this button is added also add a keyboard shortcut for it.