  We present a seven-minute long $4\pi$-3D simulation of a shell merger event
in a non-rotating $18.88\, M_\odot$ supernova progenitor before the onset of
gravitational collapse. The key motivation is to capture the large-scale mixing
and asymmetries in the wake of the shell merger before collapse using a
self-consistent approach. The $4\pi$ geometry is crucial as it allows us to
follow the growth and evolution of convective modes on the largest possible
scales. We find significant differences between the kinematic, thermodynamic
and chemical evolution of the 3D and the 1D model. The 3D model shows vigorous
convection leading to more efficient mixing of nuclear species. In the 3D case
the entire oxygen shell attains convective Mach numbers of $\mathord{\approx}\,
0.1$, whereas in the 1D model, the convective velocities are much lower and
there is negligible overshooting across convective boundaries. In the 3D case,
the convective eddies entrain nuclear species from the neon (and carbon) layers
into the deeper part of the oxygen burning shell, where they burn and power a
violent convection phase with outflows. This is a prototypical model of a
convective-reactive system. Due to the strong convection and the resulting
efficient mixing, the interface between the neon layer and the silicon-enriched
oxygen layer disappears during the evolution, and silicon is mixed far out into
merged oxygen/neon shell. Neon entrained inwards by convective downdrafts
burns, resulting in lower neon mass in the 3D model compared to the 1D model at
time of collapse. In addition, the 3D model develops remarkable large-scale,
large-amplitude asymmetries, which may have important implications for the
impending gravitational collapse and the subsequent explosion.
