  The increasingly neutral intergalactic gas at $z>6$ impacts the
Lyman-$\alpha$ flux observed from galaxies. One luminous galaxy, COLA1, stands
out because of its unique double-peaked Ly$\alpha$ line at $z=6.6$, unseen in
any simulation of reionization. Here we present JWST/NIRCam wide-field slitless
spectroscopy in a 21 arcmin$^2$ field centered on COLA1. We find 141 galaxies
spectroscopically-selected through the [OIII]($\lambda4969,5008$) doublet at
$5.35<z<6.95$, with 40 of these sources showing H$\beta$. For COLA1 we
additionally detect [OIII]$_{4363}$ and H$\gamma$. We measure a systemic
redshift of $z=6.5917$ for COLA1, confirming the double-peak nature of the
Ly$\alpha$ profile. This implies that it resides in a highly ionized bubble and
that it is leaking ionizing photons with a high escape fraction $f_{\rm
esc}{\rm (LyC)}=20$-$50$%, making it a prime laboratory to study Lyman
continuum escape in the Epoch of Reionization. COLA1 shows all the signs of a
prolific ionizer with a Ly$\alpha$ escape fraction of $81\pm5\%$, Balmer
decrement indicating no dust, a steep UV slope ($\beta_{\rm UV}=-3.2\pm 0.4$),
and a star-formation surface density $\gtrsim 10\times$ that of typical
galaxies at similar redshift. We detect 5 galaxies in COLA1's close environment
($\Delta z<0.02$). Exploiting the high spectroscopic completeness inherent to
grism surveys, and using mock simulations that mimic the selection function, we
show the that number of detected companions is very typical for a similarly
UV-bright ($M_{\rm{UV}}\sim-21.3$) galaxy; that is, the ionized bubble around
COLA1 is unlikely due to an excessively large over-density. Instead, the
measured ionizing properties suggest that COLA1 by itself might be powering the
bubble required to explain its double-peaked Ly$\alpha$ profile ($R_{\rm
ion}\approx0.7$ pMpc), with minor contribution from detected neighbours
($-17.5>M_{\rm UV}>-19.5$).
