Root cause (live capture #3 + code): GameWindow's per-part landblock shadow
registration used a synthetic part-id `entity.Id * 256u + partIndex` that
OVERFLOWS uint32 for class-prefixed landblock ids (0x40/0x80/0xC0...). The << 8
drops the prefix byte, so different-class entities sharing the low 24 bits
collide on ONE shadow part-id and Register's deregister-then-insert silently
overwrites one entity's collision geometry. Landblock 0xF682 had 23 such
collisions incl. the stair runs (0xF6822100 <- {0x40F68221, 0xC0F68221}, ...),
so 3 mid-staircase steps rendered but had no collision -> the player floats into
the hole and the (faithful) PrecipiceSlide wedge fires = the 'invisible wall
half-way up the stairs'.
Fix (Option A, retail-faithful): register each multi-part landblock entity via
ShadowObjectRegistry.RegisterMultiPart under its UNIQUE 32-bit entity.Id
(retail CPhysicsObj::add_shadows_to_cells 0x00514ae0 -> CPartArray::AddPartsShadow
- one object, a part array; no synthetic per-part id). New testable builder
ShadowShapeBuilder.FromLandblockBspParts decomposes each MeshRef.PartTransform to
local pos/rot/scale; RegisterMultiPart reconstructs the identical world placement.
Building shells stay excluded (building channel); the Setup cyl/sphere path is
unchanged (runs only when entityBsp==0, retail BSP-xor-cyl dispatch). Despawn is
landblock-scoped (RemoveLandblock by cell prefix), so the id change is safe.
Tests: ShadowRegistrationOverflowTests (overflow arithmetic; old scheme drops one;
RegisterMultiPart keeps both; builder). Issue185OutdoorStairsSeamReplayTests
(dat-free clean-climb pin). Core 2629 / App 741 green, 0 warnings.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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