docs: file #185 - local player jams half-way up outdoor stairs (house on stilts)

docs/ISSUES.md

@@ -46,6 +46,53 @@ Copy this block when adding a new issue:
 
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+## #185 — LOCAL player jams half-way up outdoor stairs (house on stilts); a jump clears it
+
+**Status:** OPEN (investigated, root cause characterised — NOT yet fixed; a fix touches the frozen collision internals → brainstorm gate first)
+**Severity:** MEDIUM
+**Filed:** 2026-07-08
+**Component:** physics / collision (building/stair BSP + step-up vs slide classification)
+
+**Description:** Running the LOCAL player up the outside stairs of a house-on-stilts, you hit
+an invisible block **half-way up** that you cannot walk through — you shuffle sideways but
+never advance. **Jumping clears it** and you continue up normally. Pre-existing (observed
+before the #184 work); unrelated to the remote de-overlap thread. Building object
+`0xf6822103`, landblock `0xf682`, jam at world ≈ (132, 77.8, 61.4), cell `0xF682002C`.
+
+**Root cause / status (from `ACDREAM_CAPTURE_RESOLVE` at the jam):** The staircase is a
+fairly uniform ~0.4 m-rise / **38.7°** flight of tilted tread faces (`n=(0,-0.62,0.78)`),
+which is under the ~49° walkable cutoff, so the player *glides* straight up it (long
+no-collision stretches; the wireframe shows discrete stacked step faces). At **one junction
+(~Z 61.4)** there is an anomalous **~52° face** (`collisionNormal=(0,0.78,0.62)`, constant
+across every stuck frame → a real stable face, NOT the #137 synthetic-negated-run-direction
+normal). 52° is just past the walkable cutoff, so the sweep treats it as a **wall**: it
+**flattens that normal to a pure-horizontal `(0,1,0)` sliding normal** (Z zeroed) and slides
+the player sideways in X, pinning Y+Z — instead of recognising it as a normal ~0.4 m **step**
+and stepping the player up onto the next tread. The jump clears it because the jump's +Z
+velocity is not in the horizontal `(0,1,0)` slide plane. The staircase continues climbable
+above the jam (more 38.7° treads → a flat porch at Z 63.6) — so it is ONE localised spot,
+not a general stair failure. Classic **#137 / building-collision-mesh junction-anomaly family**
+(TS-4 sliding-normal-provenance), stair edition.
+
+**Open question (decides mesh-artifact vs faithful):** is that 52° face **real dat geometry**
+of object `0xf6822103` or a **mesh-generation artifact** at a BSP-leaf / sub-object junction
+on our side? The capture only records treads the player STOOD on, so it can't show the jam
+face's own extent or the tread directly above it. Decisive next read: dump `0xf6822103`'s
+collision faces (`ACDREAM_DUMP_GFXOBJS`, or resolve its GfxObj/Setup and decode the BSP) and
+check for a spurious ~52° face where a 38.7° continuation should be. If the step to the next
+tread is ≤ 0.6 m (the step-up budget; it looks ~0.4 m), step-up SHOULD lift the player → bug
+in step-up-vs-wall classification for a just-over-walkable face.
+
+**Files:** `src/AcDream.Core/Physics/TransitionTypes.cs` (step-up / slide classification, the
+sliding-normal horizontalisation); building-collision mesh generation (BSP faces for
+landblock stabs). Capture apparatus: `ACDREAM_CAPTURE_RESOLVE`.
+
+**Acceptance:** the LOCAL player walks up the full outdoor staircase of the house-on-stilts
+without jumping; no sideways-slide pin at the ~Z 61.4 junction; regression pass on other
+stairs/ramps (indoor + outdoor) — no new phantom blocks.
+
+---
+
 ## #184 — [DONE 2026-07-08 · `37a94e1f`+`f51c1dff`+`e1ac56cc`+`ddb5a967`] Remote monsters overlap (arms interpenetrating) in a crowd; retail barely overlaps
 
 **Status:** CLOSED 2026-07-08 — all four slices shipped + both visual gates passed. Slices 1+3