> min_width сan be large negative number, and there are subtractions from it. It may be safer to replace the assert with something like min_width = Py_MAX(0, min_width). Or ensure that it is non-negative before calling _PyUnicode_InsertThousandsGrouping()

Looking at the code the loop seems to operate on the assumption that min_width is >= 0 in the beginning with the assert statement until both remaining and min_width are negative to break out of the loop. Applying Py_MAX(0, min_width) causes no test failures but maybe I have missed adding a case where large negative min_width might have triggered other issue.