I did something to change the way "fit on screen" behaves. So, for example, I have opened an image that is 8.02 high. When I press cmd+0, there's an inch of gray above and below the image and the ruler on the left runs from -1 to 9. How do I convince "fit on screen" to produce an image with a ruler that runs from 0 to the height of the image?
A screen shot might help.
You mention the height - is the width expanded as you expect? Except for one case, where the aspect ratio of the image exactly matches the window, fit on screen will zoom the image to fit the width or the height, whichever reaches the edges first - leaving blank bars at the top and bottom or at the sides. It has to do this otherwise it would either distort the image or zoom in further and parts would be hidden.