  We report detailed measurements of the Onsager-like planar thermal Hall
conductivity $\kappa_{xy}$ in $\alpha$-RuCl$_3$, a spin-liquid candidate of
topical interest. With the thermal current ${\bf J}_{\rm Q}$ and magnetic field
$\bf B\parallel a$ (zigzag axis), the observed $\kappa_{xy}/T$ varies strongly
with temperature $T$ (1-10 K). The results are well-described by bosonic edge
excitations which evolve to topological magnons at large $B$. Fits to
$\kappa_{xy}/T$ yield a Chern number $\sim 1$ and a band energy $\omega_1\sim$1
meV, in agreement with sharp modes seen in electron spin-resonance experiments.
The bosonic character is incompatible with half-quantization of
$\kappa_{xy}/T$.
