Slabbed

Brings the Fabric 1.21.1 line forward from 0.4.2-beta.1. The headline: where you place a block is where it stays. This alpha introduces permanent heights for new placements while keeping existing blocks on their older behavior.

New placements are permanent; existing blocks are untouched

• Every block you place from now on records its exact height at placement time and keeps it. Breaking its support, building next to it, reloading the world, or restarting the game changes nothing. This closes the long-standing family of "block pops up / snaps down later" bugs at the root instead of case by case.
• Blocks that already exist in your world keep behaving exactly as they do today. Nothing moves when you load an older world into this version, and there is no conversion step or prompt. Older blocks keep the old rules, including the old rules' quirks, until you break and re-place them.
• Placing a new block on top of an older lowered block lands it flush on that block's visible top.
• A new placement appears at its final height immediately instead of rendering flat and snapping after the server confirms it.
• Heights are stored on a sixteenth-of-a-block grid and synchronized compactly, so dense slab-supported builds no longer risk oversized chunk packets.

Deeper placement

• Side aiming and cantilever placement now work through three blocks of depth, at every sixteenth-grid height, while shallow and ordinary vanilla placement stays as it was.
• Models, selection outlines, targeting, and collision agree at stored depths, including snow, farmland and crops, beds, carpets, powder snow, turtle eggs, panes, and slab-attached blocks.
• Ambient and event particles stay attached to lowered blocks: torches, wall torches, campfires, candles, levers, decorated pots, redstone burnout, and block-display particles.
• Pistons carry a moved block's stored height with it, and a moving piston's block is no longer drawn shifted twice.

Entities on lowered blocks

• Item frames and glow item frames placed on a lowered block sit at its real height, physically and visually, so they can be targeted where you see them.
• Minecarts follow rails on lowered blocks physically; riders sit where the cart is drawn.
• Boats and armor stands placed onto a lowered block land on its visible surface.
• These apply to blocks placed in this version. On older blocks, frames and carts keep the old render-only behavior.

Fixed

• Fixed bed and shulker-box inventory icons and held rendering by preserving their builtin models.
• Fixed a world-loading and saving hang caused by chest minecarts created during world generation. Minecarts now wait for their normal server tick before checking the rails beneath them.
• Restored vanilla redstone connection, direction, occlusion, and power rules. Redstone can still use supported slab steps, but ordinary solid blocks no longer create phantom wire arms or power paths.
• Bottom slabs and compatible bottom-like slab surfaces stay mob-proof: making those surfaces usable for placement no longer makes them valid ground for mob spawning.
• Placed heights stay stable when a block changes shape or state in place: fence, wall, and pane connections, waterlogging, redstone power, and stair-shape updates.
• Stored heights are no longer misread as whole blocks by the chunk renderer, which could draw a correctly placed block far below its outline and server position.
• Fabric's model path no longer applies a height offset twice on some non-vanilla models.
• Scaffolding on a lowered column is stood on, climbed and descended at its real height instead of sliding down when jump is released.
• A Terrain Slabs slab you place records its height like any other slab, so it sits on a lowered block instead of floating. Natural Terrain Slabs terrain and slabs from older worlds are untouched.

Limits and notes

• Older blocks keep the old behavior by design; this release does not migrate existing placements.
• -Dslabbed.frozenDy=true makes every block, old or new, read its stored height (a block with no stored height reads flat). This is an opt-in for testing, not a supported way to play older worlds.
• No claim is made about compatibility with large rendering or optimization modpacks beyond the mods listed as tested in the release notes.
• Open reports about delayed render refresh, other custom item icons, and glass-pane crashes in large packs are not declared fixed by this release without their own retest.
• This is a Fabric 1.21.1 changelog. Changes released on other Minecraft versions or loaders are not its baseline.