  We obtain for the first time the analytic two-loop four-point MHV lightlike
form factor of the stress-tensor supermultiplet in planar ${\cal N}=4$ SYM
where the momentum $q$ carried by the operator is taken to be massless.
Remarkably, we find that the two-loop result can be constrained uniquely by the
infrared divergences and the collinear limits using the master-bootstrap
method. Moreover, the remainder function depends only on three dual conformal
invariant variables, which can be understood from a hidden dual conformal
symmetry of the form factor arising in the lightlike limit of $q$. The symbol
alphabet of the remainder contains only nine letters, which are closed under
the action of the dihedral group $D_4$. Based on the dual description in terms
of periodic Wilson lines (null-wrapped polygons), we also consider a new OPE
picture for the lightlike form factors and introduce a new form factor
transition that corresponds to the three-point lightlike form factor. With the
form factor results up to two loops, we make some all-loop predictions using
the OPE picture. A preliminary study at three loops using symbol bootstrap is
discussed.
