  We report the measurement of reaction cross sections ($\sigma_R^{\rm ex}$) of
$^{27,29}$F with a carbon target at RIKEN. The unexpectedly large
$\sigma_R^{\rm ex}$ and derived matter radius identify $^{29}$F as the heaviest
two-neutron Borromean halo to date. The halo is attributed to neutrons
occupying the $2p_{3/2}$ orbital, thereby vanishing the shell closure
associated with the neutron number $N = 20$. The results are explained by
state-of-the-art shell model calculations. Coupled-cluster computations based
on effective field theories of the strong nuclear force describe the matter
radius of $^{27}$F but are challenged for $^{29}$F.
