Live capture #3 (cp-write + entity-source + full bsp-test object map) disproved
v1's grounding-retention theory and pinned the real bug: GameWindow.cs:7951
partId = entity.Id*256+partIndex OVERFLOWS uint32 for class-prefixed landblock
ids (0x40/0x80/0xC0), dropping the prefix byte so different-class entities sharing
the low 24 bits collide on one shadow part-id; Register deregisters the loser
(last-writer-wins), silently deleting collision geometry while render shows every
step. Landblock 0xF682 has 23 such collisions incl. the stair runs. The player
floats into the collision hole and the PrecipiceSlide wedge fires = the invisible
wall (a faithful symptom). Fix = Option A: RegisterMultiPart per entity (unique
32-bit entity.Id, retail add_shadows_to_cells/AddPartsShadow model), unifying on
the one faithful multi-part path and deleting both synthetic-id schemes.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Root cause confirmed (decomp cross-check wf_3c1120c4-a04 + live apparatus): the
outdoor stairs are a continuous coplanar 38.7-degree ramp of stacked step-box
objects; at a seam the grounded forward move loses contact_plane_valid, the
step-down recovery can't reach the coplanar (at-level) continuation, and
EdgeSlide/PrecipiceSlide fabricates a horizontal (0,1,0) sliding normal that
absorbs the up-stairs motion (the #137/TS-4 family). Fabrication math, the
SetSlidingNormal Z-zero, and the multi-object search are all verified faithful;
the divergence is upstream (retail keeps contact_plane_valid, pc 273244).
Approach A: restore retail's grounded forward-move retention so the fragile
step-down recovery isn't needed at seams. Exact retention-loss line pinned by a
dat-backed replay test (the #137 method), not guessed. Alternatives B/C recorded.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
