Slabbed — Skill: Ceiling Support

Goal

Allow blocks that require ceiling attachment (chains, hanging signs, lanterns) to place on and survive under top slabs and double slabs.

Key finding (2026-02-07)

Analysis showed that ceiling support already works without additional mixins:

• SlabSupportStateMixin.isSideSolid returns true for Direction.DOWN on top slabs (all SideShapeType variants)
• SlabSupportBlockMixin.sideCoversSmallSquare returns true for Direction.DOWN on top slabs
• Top slab collision shape [0,0.5,0 → 1,1,1] natively satisfies isSideSolid(DOWN, CENTER) via shape cache
• ChainBlock has no canPlaceAt override — places anywhere
• HangingSignBlock.canPlaceAt checks isSideSolid(pos.up(), DOWN, CENTER) — satisfied by top slabs natively
• Lanterns use sideCoversSmallSquare(DOWN) — covered by SlabSupportBlockMixin

When to use this skill

When adding ceiling-attachment support for a new block type under top slabs.

Hard stops

• Ceiling support leaks to bottom slabs, stairs, fences, walls, or panes
• Baseline full-block behavior changes
• Build fails

Steps

1. Test whether existing isSideSolid / sideCoversSmallSquare changes already enable placement
2. If not: check what placement method the block uses (isSideSolid, sideCoversSmallSquare, or custom)
3. If custom: add a narrow per-block mixin (canPlaceAt inject with early return) routing through SlabSupport
4. Verify survival after neighbor updates (break and replace the slab above)
5. Verify visual alignment (flush against slab bottom)
6. Build gate
7. Test matrix
8. Commit + tag

Note on isCeilingSupportSurface

A dedicated SlabSupport.isCeilingSupportSurface(BlockView, BlockPos) helper was planned but is NOT needed currently. The existing shared hooks cover all known ceiling-attachment blocks. Only add this helper if a future block requires a custom check that the shared hooks don't cover.