Two possible ways to achieve this:
- Have an option to start the operation as soon as the selection is made. As soon as the selection is made, you can start to scroll the image. I don't really see the point of the start button. If I make a mistake while making the selection, I just hit <escape> and start again. Once I have made the selection, I want to start scrolling immediately. I don't see what purpose the start button serves.
- If for some reason it is necessary to have the start button, then at least have it already selected so that I can just hit return or space to activate it, rather than having to go and click it with the mouse.