Fix Shadow Corruption from Stale DynaPoly floorPoly Pointers

soh/src/code/z_actor.c

@@ -103,7 +103,19 @@ void ActorShadow_Draw(Actor* actor, Lights* lights, PlayState* play, Gfx* dlist,
     f32 temp2;
     MtxF sp60;
 
-    if (actor->floorPoly != NULL) {
+    // https://github.com/HarbourMasters/Shipwright/issues/1953
+    //
+    // Actors that cache floorPoly pointers into the DynaPoly polyList buffer can end up holding stale references when
+    // DynaPoly_ExpandSRT rebuilds the buffers.  The pointer address stays valid but the data at that
+    // address shifts to a different poly -- often a wall poly with a horizontal normal.
+    //
+    // When this happens, func_80038A28 builds the shadow matrix from the wall normal, rotating the shadow 90-degrees
+    // onto a vertical plane.  Combined with G_CULL_BACK in sSetupDL[0x2C], the shadow flickers in and out depending on
+    // camera angle.
+    //
+    // Add a normal.y > 0 guard so polys that no longer face upward are rejected.  This prevents rendering from stale
+    // wall or ceiling polys.
+    if (actor->floorPoly != NULL && actor->floorPoly->normal.y > 0) {
         temp1 = actor->world.pos.y - actor->floorHeight;
 
         if (temp1 >= -50.0f && temp1 < 500.0f) {

soh/src/code/z_bgcheck.c

@@ -2960,6 +2960,34 @@ void func_8003F8EC(PlayState* play, DynaCollisionContext* dyna, Actor* actor) {
 /**
  * DynaPolyInfo_setup
  */
+
+// https://github.com/HarbourMasters/Shipwright/issues/1953
+//
+// When DYNAPOLY_INVALIDATE_LOOKUP triggers a full rebuild in DynaPoly_ExpandSRT, the polyList buffer is rewritten and
+// poly indices can shift (i.e., when a BgActor is added or removed).  Actors that cached a floorPoly pointer into the
+// old buffer now hold stale references.
+//
+// Rederive floorPoly for every actor on a DynaPoly surface so their shadows render from correct data.
+static void DynaPoly_RefreshActorFloorPolys(PlayState* play) {
+    Actor* actor = NULL;
+    s32 floorBgId = 0;
+
+    for (s32 category = 0; category < ACTORCAT_MAX; ++category) {
+        for (actor = play->actorCtx.actorLists[category].head; actor != NULL; actor = actor->next) {
+            if (actor->floorBgId != BGCHECK_SCENE) {
+                Vec3f checkPos;
+                checkPos.x = actor->world.pos.x;
+                checkPos.y = actor->world.pos.y + 50;
+                checkPos.z = actor->world.pos.z;
+                actor->floorHeight = BgCheck_EntityRaycastFloor5(play, &play->colCtx,
+                                                                 &actor->floorPoly, &floorBgId,
+                                                                 actor, &checkPos);
+                actor->floorBgId = floorBgId;
+            }
+        }
+    }
+}
+
 void DynaPoly_Setup(PlayState* play, DynaCollisionContext* dyna) {
     DynaPolyActor* actor;
     s32 vtxStartIndex;
@@ -2999,6 +3027,9 @@ void DynaPoly_Setup(PlayState* play, DynaCollisionContext* dyna) {
             dyna->bitFlag |= DYNAPOLY_INVALIDATE_LOOKUP;
         }
     }
+
+    bool isPolyListInvalid = (dyna->bitFlag & DYNAPOLY_INVALIDATE_LOOKUP) != 0;
+
     vtxStartIndex = 0;
     polyStartIndex = 0;
     for (i = 0; i < BG_ACTOR_MAX; i++) {
@@ -3007,6 +3038,10 @@ void DynaPoly_Setup(PlayState* play, DynaCollisionContext* dyna) {
         }
     }
     dyna->bitFlag &= ~DYNAPOLY_INVALIDATE_LOOKUP;
+
+    if (isPolyListInvalid) {
+        DynaPoly_RefreshActorFloorPolys(play);
+    }
 }
 
 /**