Inclusive-cluster $\sigma^{2}_{\text{long, 5}\times\text{5}}$ distribution in data (black bullets) and PYTHIA 8 simulation (jet$-$jet $+$ $\gamma$$-$jet processes, blue squares). The four panels display these distributions for two selected cluster $p_{\rm T}$ ranges, 12 $< p_{\rm T}<  $14 GeV/$c$ on the left and 40 $< p_{\rm T}< $ 60 GeV/$c$ on the right, and two collision systems: pp (top) and Pb$-$Pb 0$-$10% central (bottom). The simulation is decomposed in its different particle origins: prompt $\gamma$ ($\gamma^{\rm prompt}$, green line), not merged decay $\gamma$ ($\gamma^{\rm decay}$, blue area), merged decay photon clusters ($\gamma\gamma$) from $\pi^{0}$ (red area) or $\eta$ (brown area) The threshold value $\sigma^{2}_{\rm long,  5\times5} =$ 0.3 or 0.392 (corresponding to $\sigma_{\rm max}^{2}$(13 GeV/$c$)) is shown on all plots as a dotted or dashed vertical line, respectively.