  We present the discovery and analysis of J1316+2614 at z=3.6130, a UV-bright
star-forming galaxy ($M_{\rm UV} \simeq -24.7$) with large escape of Lyman
continuum (LyC) radiation. J1316+2614 is a young ($\simeq 10$ Myr) star-forming
galaxy with $SFR \simeq 500 M_{\odot}$ yr$^{-1}$ and a starburst mass of
log($M_{\star}/M_{\odot}) \simeq 9.7$. It shows a very steep UV continuum,
$\beta_{\rm UV} \simeq -2.59 \pm 0.05$, consistent with residual dust
obscuration, $E(B-V)\simeq 0$. LyC emission is detected with high significance
($\simeq 17 \sigma$) down to $830$\r{A}, for which a very high relative
(absolute) LyC escape fraction $f_{\rm esc} \rm (LyC) \simeq 0.92$ ($\simeq
0.87$) is inferred. The contribution of a foreground or AGN contamination to
the LyC signal is discussed but is unlikely. J1316$+$2614 is the most powerful
ionizing source known among the star-forming galaxy population, both in terms
of production ($Q_{\rm H} \approx 10^{56}$ s$^{-1}$) and escape of ionizing
photons ($f_{\rm esc} \rm (LyC) \approx 0.9$). Nebular emission in Ly$\alpha$,
H$\beta$, and other rest-frame optical lines are detected, but these are weak
($EW_{0} \rm [H\beta] \simeq 35$\r{A}), with their strengths reduced roughly by
$\simeq 90\%$. J1316+2614 is the first case known where the effect of large
escape of ionizing photons on the strength of nebular lines and continuum
emission is clearly observed. Gas inflows are detected in J1316+2614 from the
blue-dominated peak Ly$\alpha$ emission (with a blue-to-red peak line ratio
$I_{\rm blue}/I_{\rm red} \simeq 3.7$) and redshifted ISM absorption ($\simeq
100$ km s$^{-1}$). Our results suggest that J1316+2614 is undergoing a gas
compaction event, possibly representing a short-lived phase in the evolution of
massive and compact galaxies, where strong gas inflows have triggered an
extreme star formation episode and nearly $100\%$ LyC photons are escaping.
