In scripts that use elongation (Arabic,
Mongolian, Syriac, etc.), this flag signifies
that it is safe to insert a U+0640 TATWEEL
character before this cluster for elongation.
Indicates that if input text is broken at the
beginning of the cluster this glyph is part of,
then both sides need to be re-shaped, as the
result might be different.
Indicates that if input text is changed on one side
of the beginning of the cluster this glyph is part
of, then the shaping results for the other side
might change.