  The Kitaev quantum spin liquid (QSL) on the two-dimensional honeycomb lattice
epitomizes an entangled topological state, where the spins fractionalize into
Majorana fermions. This state has aroused tremendous interest because it
harbors non-Abelian anyon excitations. The half-integer quantized thermal Hall
(HIQTH) conductance observed in $\alpha$-RuCl$_3$ is a key signature of these
excitations. However, the fate of this topologically nontrivial state at
intense fields remains largely elusive. Here, we report the thermal
conductivity $\kappa$ and specific heat $C$ of $\alpha$-RuCl$_3$ with in-plane
magnetic fields $H$. For the field direction perpendicular to the Ru-Ru bond,
where the HIQTH effect is observed, we find a discontinuous jump in $\kappa(H)$
and a peak anomaly in $C(H)$ at $\mu_0H^*\approx11$\,T, evidencing a weak
first-order phase transition. Remarkably, the HIQTH effect vanishes close to
$H^*$. Furthermore, we find that the spin-fractionalization feature is retained
well above $H^\ast$. These imply the emergence of the phase transition that
separates two QSL phases with distinct topological properties.
