Slabbed

Hotfix for 26.2. Version note: alpha.11 is the next free number in the shared alpha series (8 and 9 are held by the Fabric 1.21.1 line).

A lowered slab that changes kind in place keeps its height. A Terrain Slabs grass slab turns into a dirt slab when you cover it, the way vanilla grass turns to dirt. On a lowered stack the converted slab popped back up to grid height and sank into the block above it, because the change of block kind was treated like the slab leaving its cell. A block that changes kind but keeps its shape is the same thing you placed, so it now keeps its height. A change of shape, a slab becoming a carpet or a full block becoming a slab, still counts as a new block.

Item frames and paintings hang where you see the block, and remember it. A frame or painting placed on a lowered block's visible face hung at normal grid height, floating above (or beside) the surface it was aimed at. Both now hang on the drawn face, and both remember the height of the face they were hung on: it is decided once, when you hang them, saved with the world, and never re-read from the wall. Rebuilding the wall behind a frame at a different height moves the wall, not the frame. Frames and paintings from an older save take their height from their wall once, the first time they load, and remember it from then on.

Wall signs were checked too: a wall sign hangs on its wall and is never pulled down by a slab on the floor under it. An automated check for that scene now runs on every Minecraft version Slabbed supports. Thanks to ciambos on CurseForge and lochy-hash on GitHub for reporting this (GitHub issue #48).