Significant experimental and theoretical activity at the LHC is dedicated to the study of an elusive state of matter, the quark-gluon plasma, created in head-on heavy-ion collisions. This plasma has been called the most perfect fluid given its small shear viscosity-to-entropy ratio. Measurements of fully reconstructed jets in these collisions allow to study new aspects of this exotic state via its coupling to perturbative degrees of freedom. We discuss how energetic colour charges propagate in a quark-gluon plasma and extend the previously known formalism to compute energy-loss of independent partons to treat neighbouring partons within the same jet. This extension accounts for the coherent structure of jet fragmentation and implies subtle modifications of the substructure. As a concrete application, we discuss the expected medium-modification of the jet splitting probability that is measured using the SoftDrop procedure.