  In a recent paper by K.-H. Lee and K. Lee, rigid reflections are defined for
any Coxeter group via non-self-intersecting curves on a Riemann surface with
labeled curves. When the Coxeter group arises from an acyclic quiver, the rigid
reflections are related to the rigid representations of the quiver. For a
family of rank $3$ Coxeter groups, it was conjectured in the same paper that
there is a natural bijection from the set of reduced positive roots of a
symmetric rank $2$ Kac--Moody algebra onto the set of rigid reflections of the
corresponding rank $3$ Coxeter group. In this paper, we prove the conjecture.
