  Event classifiers based either on the charged-particle multiplicity or on
event topologies, such as spherocity and underlying event, became very useful
tools to study collective-like behaviors in small collision systems. However,
multiplicity-based event classifiers were shown to bias the data sample in a
way that can obscure the effects of multi-parton interactions, and, this way,
make it difficult to pin down the origins of small-system collectivity.
  In this proceedings, the measurement of the transverse momentum
($p_{\mathrm{T}}$) spectra of primary charged pions, kaons, (anti)protons and
unidentified hadrons in inelastic pp collisions at $\sqrt{s}=13~\mathrm{TeV}$
are reported. Events are classified using a novel event shape observable,
flattenicity, that was proposed to select minijet-enhanced pp collisions.
Particle production is studied as a function of flattenicity and
double-differentially as a function of flattenicity and charged-particle
multiplicity. The results are compared with theoretical predictions from the
PYTHIA 8 and EPOS LHC Monte Carlo models.
