fix: never drop walkthrough player below the site ground plane

The site ground collider was sized to the scene footprint with a 30 m
minimum — exactly the default site polygon, so one step past the site
boundary left nothing under the character controller and the player
fell into the void. The ground collider now extends 2 km (still a
single BVH box) so the ground plane acts unbounded, and a respawn net
in the first-person frame loop recovers the controller if it ever ends
up below every collider (e.g. scenes with no site node), preferring the
live spawn node over the mount-time start position.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/editor/src/components/editor/first-person-controls.tsx

@@ -78,6 +78,7 @@ const ELEVATOR_COLLIDER_FLOOR_THICKNESS = 0.08
 const ELEVATOR_COLLIDER_DOOR_DEPTH = 0.12
 const ELEVATOR_ENTRY_DOOR_OPEN_THRESHOLD = 0.72
 const DEFAULT_ELEVATOR_LEVEL_HEIGHT = 2.5
+const VOID_FALL_RESPAWN_DEPTH = 12
 const keyboardMap = [
   { name: 'forward', keys: ['ArrowUp', 'KeyW'] },
   { name: 'backward', keys: ['ArrowDown', 'KeyS'] },
@@ -1217,6 +1218,29 @@ export const FirstPersonControls = () => {
     if (!controllerRef.current?.group) return
 
     const group = controllerRef.current.group
+
+    // The site ground collider is effectively unbounded, but scenes without a
+    // site node only have finite fallback floors — if the controller still ends
+    // up below every collider it can never land, so put it back at the spawn.
+    // Prefer the live spawn node over the mount-time start position so a spawn
+    // moved mid-walkthrough doesn't respawn the player at stale coordinates.
+    const worldBounds = worldRef.current?.bounds
+    if (worldBounds && group.position.y < worldBounds.min.y - VOID_FALL_RESPAWN_DEPTH) {
+      const respawnPosition = placedSpawn
+        ? [
+            placedSpawn.position[0],
+            placedSpawn.position[1] - CONTROLLER_CENTER_FROM_EYE,
+            placedSpawn.position[2],
+          ]
+        : controllerStart?.position
+      if (respawnPosition) {
+        group.position.set(respawnPosition[0]!, respawnPosition[1]!, respawnPosition[2]!)
+        controllerRef.current.resetLinVel()
+        ridingElevatorRef.current = null
+        setElevatorRideLocked(false)
+      }
+    }
+
     group.rotation.y = 0
     camera.position.copy(group.position).add(cameraOffset)
     cameraEuler.set(pitchRef.current, yawRef.current, 0, 'YXZ')

packages/editor/src/components/editor/first-person/build-collider-world.test.ts

@@ -141,9 +141,12 @@ describe('buildFirstPersonColliderWorldFromRegistry', () => {
     // Ground slab sits just below the site ground plane (y = 0).
     expect(world?.bounds?.min.y).toBeCloseTo(-0.08)
     expect(world?.bounds?.max.y).toBeCloseTo(0)
-    // Default site footprint falls back to the 30 m minimum size.
-    expect(world?.bounds?.min.x).toBeCloseTo(-15)
-    expect(world?.bounds?.max.x).toBeCloseTo(15)
+    // The ground collider extends far past the site polygon so stepping out of
+    // the site boundary never drops the player below the ground plane.
+    expect(world?.bounds?.min.x).toBeCloseTo(-1000)
+    expect(world?.bounds?.max.x).toBeCloseTo(1000)
+    expect(world?.bounds?.min.z).toBeCloseTo(-1000)
+    expect(world?.bounds?.max.z).toBeCloseTo(1000)
     world?.dispose()
   })
 })

packages/editor/src/components/editor/first-person/build-collider-world.ts

@@ -27,6 +27,7 @@ const OPERATION_DOOR_COLLIDER_OPEN_THRESHOLD = 0.85
 const LEVEL_FALLBACK_FLOOR_THICKNESS = 0.08
 const LEVEL_FALLBACK_FLOOR_PADDING = 2
 const LEVEL_FALLBACK_FLOOR_MIN_SIZE = 30
+const SITE_GROUND_COLLIDER_MIN_SIZE = 2000
 
 export const FIRST_PERSON_SPAWN_EYE_HEIGHT = SPAWN_EYE_HEIGHT
 
@@ -129,8 +130,10 @@ function collectLevelFallbackFloorGeometries(nodes: SceneNodes) {
 // a dedicated collider, a spawn on the bare ground (no slab, or not parented to
 // a level that triggers the per-level fallback) has no floor to stand on and the
 // walkthrough player falls through. Derive a thin ground slab from node data (not
-// the rendered mesh) so it exists regardless of geometry-mount timing, sized to
-// cover the whole scene footprint at the site's ground plane.
+// the rendered mesh) so it exists regardless of geometry-mount timing. The slab
+// is effectively unbounded (not sized to the site polygon): the ground plane must
+// keep holding the player up even after they step past the site boundary,
+// otherwise they fall below the ground plane into the void.
 function createSiteGroundColliderGeometry(site: SiteNode, nodes: SceneNodes) {
   if (site.visible === false) return null
 
@@ -142,11 +145,11 @@ function createSiteGroundColliderGeometry(site: SiteNode, nodes: SceneNodes) {
   const [boundsWidth, boundsDepth] = bounds?.size ?? [0, 0]
   const width = Math.max(
     boundsWidth + LEVEL_FALLBACK_FLOOR_PADDING * 2,
-    LEVEL_FALLBACK_FLOOR_MIN_SIZE,
+    SITE_GROUND_COLLIDER_MIN_SIZE,
   )
   const depth = Math.max(
     boundsDepth + LEVEL_FALLBACK_FLOOR_PADDING * 2,
-    LEVEL_FALLBACK_FLOOR_MIN_SIZE,
+    SITE_GROUND_COLLIDER_MIN_SIZE,
   )
 
   const geometry = createBoxColliderGeometry(width, LEVEL_FALLBACK_FLOOR_THICKNESS, depth)