Not really something you missed but if I want to increase the “viewport” of an image, I just take a screenshot (to clipboard), create a new document, paste the screenshot, and then go back to my original image and copy/paste it to the new document.

This is helpful if you want to extend an image horizontally or vertically (or both).

You could also increase the dimensions of the original image, but it would “zoom in”, which I don’t want.

Not sure I explained it well…