The specific bug being fixed here is that wrapped lines before newline are ignored because after 

        if s[pos] == '\n':
            linecount += 1
            current_column = 0

this block

        if current_column > 0:
            lines, column = divmod(current_column - 1, linewidth)
            linecount += lines
            current_column = column + 1

has no effect.  The fix, in the PR, is to put the linecount increment in the \n block before resetting current_column.  (Since the column>0 block also has no net effect after \t, I removed it)