  While the existence of a magnetic field induced quantum spin liquid in Kitaev
magnets remains under debate, its topological properties often extend to
proximal phases where they can lead to unusual behaviors of both fundamental
and applied interests. Subjecting a generic nearest neighbor spin model of
Kitaev magnets to a sufficiently strong in-plane magnetic field, we study the
resulting polarized phase and the associated magnon excitations. In contrast to
the case of an out-of-plane magnetic field where the magnon band topology is
enforced by symmetry, we find that it is possible for topologically trivial and
nontrivial parameter regimes to coexist under in-plane magnetic fields. We map
out the topological phase diagrams of the magnon bands, revealing a rich
pattern of variation of the Chern number over the parameter space and the field
angle. We further compute the magnon thermal Hall conductivity as a weighted
summation of Berry curvatures, and discuss experimental implications of our
results to planar thermal Hall effects in Kitaev magnets.
