Close the image window. As GIMP prompts you to save it, swear again. Why make a fork? The GIMP developers have stated many times already that they don't intend to revert the changes or even to add an option to facilitate the non-XCF workflow, and recommended to use another application ...
For example, one website might use square angular edges, but another might use rounded corners – so you can adapt to use rounded corners in the background image by changing 1 line of the generation script. Mice are evil –why involve a mouse when all we need is a keyboard? To install...