Evolution of the jet opening angle in the two-dimensional simulation shown in Figure 7. The opening angle is defined as the angle enclosing 75% and 95% of the total isotropic-equivalent jet energy (E_iso = 4 × 10⁵³ erg). The opening angle containing 75% of the jet energy remains nearly constant throughout most of the evolution, demonstrating that the relativistic core undergoes very little lateral expansion. In contrast, the opening angle enclosing 95% of the energy gradually increases on timescales comparable to the Blandford–McKee deceleration time, t_d,BM, reflecting the lateral expansion of the lower-energy material surrounding the jet core.