The cubby column divider was full-depth while the boards are depth-recessed,
so it (a) sat proud of the shelf fronts — the earlier Y-tuck then made it poke
past the top/bottom boards — and (b) shared the back panel's rear plane,
z-fighting down the centre of the back.
Give the divider the same depth recess as the boards (via boardGeometry) and
drop the Y-tuck: it now sits flush with the shelf fronts and its back tucks
inside the back panel. Flush top/bottom is fine — those board faces are
back-to-back, not co-facing, so they don't fight.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
