Caret x for each character boundary start..=end (len = end - start + 1), downstream: the
leading edge of the character after the boundary (its right edge if it is right-to-left);
the last entry is the trailing edge of the line’s last character.
Visual box (left, right) of each character start..end. Characters inside a grapheme have
empty boxes; a ligature’s characters share its width (GDEF ligature carets).
Visual x intervals covering paragraph characters from..to on this line, left to right
and merged where they touch. Bidi text can make a logical range several boxes.