If the image is a single colour I should not need to change the tolerance. What you're doing is changing it so it overwrites the whole image.
Try this instead, make a new single colour image and then paint one corner a different colour. Now try and use the paint bucket to change the original coloured area to something else. You'll get the same artifact in the three un-recoloured corners unless you drag out to change the tolerance enough to also overwrite the corner whose colour you've changed.
Smooth edges is broken. It shouldn't be finding non-existant edges in a solid-colour image.
Try this instead, make a new single colour image and then paint one corner a different colour. Now try and use the paint bucket to change the original coloured area to something else. You'll get the same artifact in the three un-recoloured corners unless you drag out to change the tolerance enough to also overwrite the corner whose colour you've changed.
Smooth edges is broken. It shouldn't be finding non-existant edges in a solid-colour image.