  A search for long-lived charginos produced either directly or in the cascade
decay of heavy prompt gluino states is presented. The search is based on
proton-proton collision data collected at a centre-of-mass energy of $\sqrt{s}$
= 13 TeV between 2015 and 2018 with the ATLAS detector at the LHC,
corresponding to an integrated luminosity of 136 fb$^{-1}$. Long-lived
charginos are characterised by a distinct signature of a short and then
disappearing track, and are reconstructed using at least four measurements in
the ATLAS pixel detector, with no subsequent measurements in the
silicon-microstrip tracking volume nor any associated energy deposits in the
calorimeter. The final state is complemented by a large missing
transverse-momentum requirement for triggering purposes and at least one
high-transverse-momentum jet. No excess above the expected backgrounds is
observed. Exclusion limits are set at 95% confidence level on the masses of the
chargino and gluino for different chargino lifetimes. Chargino masses up to 660
(210) GeV are excluded in scenarios where the chargino is a pure wino
(higgsino). For charginos produced during the cascade decay of a heavy gluino,
gluinos with masses below 2.1 TeV are excluded for a chargino mass of 300 GeV
and a lifetime of 0.2 ns.
