  We report measurements of charm-mixing parameters based on the
decay-time-dependent ratio of $D^0\to K^+\pi^-$ to $D^0\to K^-\pi^+$ rates. The
analysis uses a data sample of proton-proton collisions corresponding to an
integrated luminosity of $5.0$ fb$^{-1}$ recorded by the LHCb experiment from
2011 through 2016. Assuming charge-parity (CP) symmetry, the mixing parameters
are determined to be $x'^2=(3.9 \pm 2.7) \times10^{-5}$, $y'=(5.28 \pm 0.52)
\times 10^{-3}$, and $R_D=(3.454 \pm 0.031)\times10^{-3}$. Without this
assumption, the measurement is performed separately for $D^0$ and
$\overline{D}{}^0$ mesons, yielding a direct CP-violating asymmetry $A_D
=(-0.1\pm9.1)\times10^{-3}$, and magnitude of the ratio of mixing parameters
$1.00< |q/p| <1.35$ at the $68.3\%$ confidence level. All results include
statistical and systematic uncertainties and improve significantly upon
previous single-measurement determinations. No evidence for CP violation in
charm mixing is observed.
