  The CKM matrix element $|V_{us}|$ can be extracted from the experimental
measurement of semileptonic $K\to\pi$ decays. The determination depends on
theory input for the corresponding vector form factor in QCD. We present a
preliminary update on our efforts to compute it in $N_f=2+1$ lattice QCD using
domain wall fermions for several lattice spacings and with a lightest pion mass
of about $170\,\mathrm{MeV}$. By using partially twisted boundary conditions we
avoid systematic errors associated with an interpolation of the form factor in
momentum-transfer, while simulated pion masses near the physical point reduce
the systematic error due to the chiral extrapolation.
