  A measurement of $CP$-violating observables is performed using the decays
$B^\pm\to D K^\pm$ and $B^\pm\to D \pi^\pm$, where the $D$ meson is
reconstructed in one of the self-conjugate three-body final states $K_{\mathrm
S}\pi^+\pi^-$ and $K_{\mathrm S}K^+K^-$ (commonly denoted $K_{\mathrm S}
h^+h^-$). The decays are analysed in bins of the $D$-decay phase space, leading
to a measurement that is independent of the modelling of the $D$-decay
amplitude. The observables are interpreted in terms of the CKM angle $\gamma$.
Using a data sample corresponding to an integrated luminosity of
$9\,\text{fb}^{-1}$ collected in proton-proton collisions at centre-of-mass
energies of $7$, $8$, and $13\,\text{TeV}$ with the LHCb experiment, $\gamma$
is measured to be $\left(68.7^{+5.2}_{-5.1}\right)^\circ$. The hadronic
parameters $r_B^{DK}$, $r_B^{D\pi}$, $\delta_B^{DK}$, and $\delta_B^{D\pi}$,
which are the ratios and strong-phase differences of the suppressed and
favoured $B^\pm$ decays, are also reported.
