Load up an image. Sometimes the image is at about 50% window size in both dimensions when it could be full size. Furthermore, sometimes the zoom function doesn't work, and it stays downsized even if clicked on. To break the zoom I found resizing the window to be small, refreshing and then making the window big causes it to break.
I think this behaves as designed: the zoom function never scales an image up. It is available only when the viewport is smaller than the image's native size, and then it toggles between full size and shrink-to-fit.
Sorry I wasn't clear on that. Sometimes the image is scaled down even though there is room for it at 100% and when you click on it to make it zoom to 100% it doesn't do anything even though the magnify cursor appears. It should just match the behavior of Safari which it does not.
Works for me with r56184