I followed a tutorial to perform a polar distortion on a panoramic image. The instructions are geared for Photoshop but seem to mostly apply to GIMP as well. The only thing I couldn't really figure out was how they were able to automatically fill in the area around the circle by "extending" the border of the circle. e.g. In GIMP, performing the polar distortion leaves a black white canvas around the circle, not the attractive blended background shown in the tutorial. Is there an easy way to implement this?

The only way I found was to reserve half of the "square" as blank canvas and then manually copy the image's top row of pixels over this empty portion. Then, after the polar distortion, I crop out the extra area. Although this achieves the effect, it seems a bit awkward. How do you stretch selections? Ideally, I just want to select the top row and stretch it vertically until it fills in half of the canvas. Instead I had to manaully copy, paste, translate, etc.