  A measurement of the jet mass distribution in hadronic decays of
Lorentz-boosted top quarks is presented. The measurement is performed in the
lepton+jets channel of top quark pair production ($\mathrm{t\bar{t}}$) events,
where the lepton is an electron or muon. The products of the hadronic top quark
decay are reconstructed using a single large-radius jet with transverse
momentum greater than 400 GeV. The data were collected with the CMS detector at
the LHC in proton-proton collisions and correspond to an integrated luminosity
of 138 fb$^{-1}$. The differential $\mathrm{t\bar{t}}$ production cross section
as a function of the jet mass is unfolded to the particle level and is used to
extract the top quark mass. The jet mass scale is calibrated using the hadronic
W boson decay within the large-radius jet. The uncertainties in the modelling
of the final state radiation are reduced by studying angular correlations in
the jet substructure. These developments lead to a significant increase in
precision, and a top quark mass of 173.06 $\pm$ 0.84 GeV.
