What do you expect to happen if you do a flood fill when there isn't a boundary rectangle? (legitimate question, I don't use any of these tools) My assumption

What do you expect to happen if you do a flood fill when there isn't a boundary rectangle? (legitimate question, I don't use any of these tools) My assumption is that there is at least an image boundary, and just GIMP allows layers to have a different boundary to the image. But if I didn't allow this, then every time I moved a layer, it would get cropped to the image boundary. How does not having a layer boundary work?