| name | slabbed-ceiling-support |
| description | Add hanging/ceiling attachment support for blocks under top slabs, using SlabSupport.isCeilingSupportSurface as the single source of truth. |
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 anywhereHangingSignBlock.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
- Test whether existing
isSideSolid / sideCoversSmallSquare changes already enable placement
- If not: check what placement method the block uses (
isSideSolid, sideCoversSmallSquare, or custom)
- If custom: add a narrow per-block mixin (
canPlaceAt inject with early return) routing through SlabSupport
- Verify survival after neighbor updates (break and replace the slab above)
- Verify visual alignment (flush against slab bottom)
- Build gate
- Test matrix
- 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.
