Comment 3 for bug 671508

This is caused by antialiasing, which is the default. You can turn it off in the tool toolbar.

However, the eraser tool was not respecting the antialias setting, so it still won't work. :/

Fixed in https://github.com/jpobst/Pinta/commit/06ebf2f26d80c8e702270b8fdd0c714b5316ef7e.