How do I fuse a selection to the side of a picture to effectively length it?

[marigolden]

I want to add length to a picture without stretching it, so I want to copy a sliver of it and add it onto the end. I figured out how to increase the width of the canvas size, but when I drag my selection to cover the now empty sliver of the canvas, it starts disappearing as it slips onto the checkered portion ("alpha channel"). How do I make it where it will show up on the checkerboard part so I can position it correctly and then fuse it (which I don't know how to do either so might as well ask too)