inconsistent behavior of `lapce_core::word::WordCursor::next_boundary()`

In these tests, given test_1 and test_2, I'd expect test_3 to pass. But it does not. I have to write test_4. It is also suprising that, before returning None, the method returns buffer::len() instead of buffer::len() - 1 (line A and B).

mod test {
    use crate::buffer::Buffer;
    use super::WordCursor;

    #[test]
    fn test_1() {
        let buffer = Buffer::new("a\nb");
        let mut cursor = WordCursor::new(buffer.text(), 0);
        assert_eq!(cursor.next_boundary(), Some(2));
        assert_eq!(cursor.next_boundary(), Some(buffer.len())); // <- A
        assert_eq!(cursor.next_boundary(), None);
    }

    #[test]
    fn test_2() {
        let buffer = Buffer::new("a b   ");
        let mut cursor = WordCursor::new(buffer.text(), 0);
        assert_eq!(cursor.next_boundary(), Some(2));
        assert_eq!(cursor.next_boundary(), Some(buffer.len())); // <- B
        assert_eq!(cursor.next_boundary(), None);
    }

    // This test fails.
    #[test]
    fn test_3() {
        let buffer = Buffer::new("a\nb\n\n\n");
        let mut cursor = WordCursor::new(buffer.text(), 0);
        assert_eq!(cursor.next_boundary(), Some(2));
        assert_eq!(cursor.next_boundary(), Some(buffer.len()));
        assert_eq!(cursor.next_boundary(), None);
    }

    #[test]
    fn test_4() {
        let buffer = Buffer::new("a\nb\n\n\n");
        let mut cursor = WordCursor::new(buffer.text(), 0);
        assert_eq!(cursor.next_boundary(), Some(2));
        // This assertion also fails:
        // assert_eq!(cursor.next_boundary(), Some(3));
        assert_eq!(cursor.next_boundary(), Some(4));
        assert_eq!(cursor.next_boundary(), Some(5));
        assert_eq!(cursor.next_boundary(), Some(6));
        assert_eq!(cursor.next_boundary(), None);
    }
}

Start at the lapce_core::word::WordCursor::next_boundary() entry point and run the four supplied tests to reproduce the differing boundary behavior. Trace how the cursor advances through trailing newlines and reaches buffer::len(); done means the intended sequence is consistent and the relevant tests pass.