fix(walls): bound wall miters, heal corrupt scenes, follow grid snap

Three related editor fixes surfaced while debugging a captured house
project that rendered an infinite wall and failed to load.

Infinite wall (core/systems/wall/wall-mitering.ts):
  Junction miters are line-line intersections, so the joint point sits
  ~halfThickness/sin(theta) from the junction. The only guard was an
  exact-parallel check (det < 1e-9), so two walls meeting at a shallow
  angle (a room-preset preview dragged onto an existing wall, or a wall
  drawn nearly collinear to its neighbour) produced a joint point far
  away — an infinite spike. Add a miter limit: reject joints farther
  than 10x half-thickness from the junction and fall back to a square
  joint, exactly like the parallel case.

Scene load failure (core/utils/heal-scene-graph.ts + validate-build-json
+ use-scene migrateNodes):
  Capture wall-merge could leave a `children: [null]` entry (see the
  matching merge-walls.ts fix in private-editor) and zero-length walls.
  `null` children fail wall schema validation, so the whole scene fails
  to load. Add a shared heal step — strip non-string child refs, drop
  childless zero-length walls — run on every load path: import
  validation now repairs instead of hard-failing (with a warning), and
  setScene heals on the prod project-load path too.

Grid snap (nodes slab/ceiling/spawn tools):
  These tools hardcoded a 0.5 m snap (Math.round(x*2)/2) and ignored the
  editor's grid-snap setting, so the cursor jumped by 0.5 while later
  vertices already followed the configured step. Route them through
  snapPointToGrid / snapScalar with gridSnapStep.

Adds unit tests for the miter limit and the heal step.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/core/src/store/use-scene.ts

@@ -21,6 +21,7 @@ import { SiteNode } from '../schema/nodes/site'
 import { StairNode as StairNodeSchema } from '../schema/nodes/stair'
 import { StairSegmentNode as StairSegmentNodeSchema } from '../schema/nodes/stair-segment'
 import type { AnyNode, AnyNodeId } from '../schema/types'
+import { healSceneNodes } from '../utils/heal-scene-graph'
 import * as nodeActions from './actions/node-actions'
 import { resetSceneHistoryPauseDepth } from './history-control'
 
@@ -414,7 +415,10 @@ function migrateRoofSurfaceMaterials(node: Record<string, any>) {
 }
 
 function migrateNodes(nodes: Record<string, any>): Record<string, AnyNode> {
-  const patchedNodes = { ...nodes }
+  // Repair pre-existing corruption (null children, zero-length walls) before
+  // any per-type migration runs, so already-saved scenes load cleanly.
+  const { nodes: healed } = healSceneNodes(nodes)
+  const patchedNodes = { ...healed } as Record<string, any>
   for (const [id, node] of Object.entries(patchedNodes)) {
     // 1. Item scale migration
     if (node.type === 'item' && !('scale' in node)) {

packages/core/src/systems/wall/wall-mitering.test.ts

@@ -0,0 +1,66 @@
+import { describe, expect, test } from 'bun:test'
+import type { WallNode } from '../../schema'
+import { calculateLevelMiters, getWallMiterBoundaryPoints } from './wall-mitering'
+
+function wall(id: string, start: [number, number], end: [number, number]): WallNode {
+  return {
+    id,
+    type: 'wall',
+    object: 'node',
+    visible: true,
+    parentId: 'level_test',
+    children: [],
+    start,
+    end,
+    thickness: 0.1,
+    height: 2.5,
+    frontSide: 'interior',
+    backSide: 'exterior',
+    metadata: {},
+  } as WallNode
+}
+
+function maxBoundaryCoord(walls: WallNode[]): number {
+  const miter = calculateLevelMiters(walls)
+  let max = 0
+  for (const w of walls) {
+    const bp = getWallMiterBoundaryPoints(w, miter)
+    expect(bp).not.toBeNull()
+    if (!bp) continue
+    for (const p of [bp.startLeft, bp.startRight, bp.endLeft, bp.endRight]) {
+      expect(Number.isFinite(p.x)).toBe(true)
+      expect(Number.isFinite(p.y)).toBe(true)
+      max = Math.max(max, Math.abs(p.x), Math.abs(p.y))
+    }
+  }
+  return max
+}
+
+describe('wall mitering miter limit', () => {
+  // Two 3 m walls sharing the origin, meeting at decreasing angles. Without a
+  // miter limit the joint point runs to infinity as the angle → 0 (∝ 1/sin θ),
+  // which is the "infinite wall" seen when a room-preset preview lands on top of
+  // an existing wall. The boundary must stay bounded near the wall length.
+  test.each([90, 30, 10, 5, 1, 0.1, 0.01])('stays bounded at a %s° junction', (deg) => {
+    const rad = (deg * Math.PI) / 180
+    const walls = [
+      wall('A', [0, 0], [3, 0]),
+      wall('B', [0, 0], [3 * Math.cos(rad), 3 * Math.sin(rad)]),
+    ]
+    // 3 m walls + a few cm of joint: anything past ~4 m is a runaway spike.
+    expect(maxBoundaryCoord(walls)).toBeLessThan(4)
+  })
+
+  test('still miters a normal 90° corner', () => {
+    const walls = [wall('A', [0, 0], [3, 0]), wall('B', [0, 0], [0, 3])]
+    const miter = calculateLevelMiters(walls)
+    const bpA = getWallMiterBoundaryPoints(walls[0]!, miter)
+    expect(bpA).not.toBeNull()
+    if (!bpA) throw new Error('expected miter boundary points')
+    // The shared corner is pulled to the mitred intersection, offset from the
+    // raw butt position (halfThickness 0.05) by the diagonal of the joint.
+    const startSideX = Math.min(bpA.startLeft.x, bpA.startRight.x)
+    expect(startSideX).toBeLessThan(-0.001)
+    expect(startSideX).toBeGreaterThan(-0.5)
+  })
+})

packages/core/src/systems/wall/wall-mitering.ts

@@ -35,6 +35,17 @@ type JunctionData = Map<string, WallIntersections>
 
 const TOLERANCE = 0.001
 
+// Miter joints are line-line intersections, so the joint point sits a distance
+// ≈ halfThickness / sin(θ) from the junction, where θ is the angle between the
+// two walls. As θ → 0 (two walls nearly collinear — e.g. a room-preset preview
+// dragged on top of an existing wall, or a freshly-drawn wall almost parallel
+// to its neighbour) that distance runs away to infinity and the wall renders as
+// an infinite spike. Cap the joint at this multiple of the wall half-thickness;
+// beyond it we fall back to a square (butt) joint, exactly like the existing
+// parallel-walls guard. 10× preserves every realistic corner (a 0.1 m wall keeps
+// mitering down to ~11°) while bounding the pathological near-collinear case.
+const MITER_LIMIT = 10
+
 function pointToKey(p: Point2D, tolerance = TOLERANCE): string {
   const snap = 1 / tolerance
   return `${Math.round(p.x * snap)},${Math.round(p.y * snap)}`
@@ -198,6 +209,7 @@ interface ProcessedWall {
   edgeA: LineEquation // Left edge
   edgeB: LineEquation // Right edge
   isPassthrough: boolean // True if wall passes through junction (T-junction)
+  halfThickness: number // Used to bound the miter joint against runaway spikes
 }
 
 function calculateJunctionIntersections(
@@ -228,7 +240,14 @@ function calculateJunctionIntersections(
         const edgeB = createLineFromPointAndVector(pB, v)
         const angle = Math.atan2(v.y, v.x)
 
-        processedWalls.push({ wallId: wall.id, angle, edgeA, edgeB, isPassthrough: true })
+        processedWalls.push({
+          wallId: wall.id,
+          angle,
+          edgeA,
+          edgeB,
+          isPassthrough: true,
+          halfThickness: halfT,
+        })
       }
     } else {
       // Normal wall endpoint (start or end)
@@ -245,7 +264,14 @@ function calculateJunctionIntersections(
       const edgeB = createLineFromPointAndVector(pB, v)
       const angle = Math.atan2(v.y, v.x)
 
-      processedWalls.push({ wallId: wall.id, angle, edgeA, edgeB, isPassthrough: false })
+      processedWalls.push({
+        wallId: wall.id,
+        angle,
+        edgeA,
+        edgeB,
+        isPassthrough: false,
+        halfThickness: halfT,
+      })
     }
   }
 
@@ -275,6 +301,21 @@ function calculateJunctionIntersections(
       y: (wall2.edgeB.a * wall1.edgeA.c - wall1.edgeA.a * wall2.edgeB.c) / det,
     }
 
+    // Miter limit: `det` only catches walls that are *exactly* parallel. Two
+    // walls meeting at a shallow angle have a small-but-nonzero `det`, so `p`
+    // lands far from the junction (∝ 1/sin θ) and the wall renders as an
+    // infinite spike. Reject any joint farther than MITER_LIMIT half-thicknesses
+    // from the meeting point — those walls fall back to a square joint.
+    const maxMiter = MITER_LIMIT * Math.max(wall1.halfThickness, wall2.halfThickness)
+    const dx = p.x - meetingPoint.x
+    const dy = p.y - meetingPoint.y
+    if (
+      !(Number.isFinite(p.x) && Number.isFinite(p.y)) ||
+      dx * dx + dy * dy > maxMiter * maxMiter
+    ) {
+      continue
+    }
+
     // Only assign intersection to non-passthrough walls
     // Passthrough walls don't receive junction data (their geometry doesn't change)
     if (!wall1.isPassthrough) {

packages/core/src/utils/heal-scene-graph.test.ts

@@ -0,0 +1,50 @@
+import { describe, expect, test } from 'bun:test'
+import { healSceneNodes } from './heal-scene-graph'
+
+describe('healSceneNodes', () => {
+  test('strips non-string (null) children entries', () => {
+    const { nodes, strippedChildRefs } = healSceneNodes({
+      wall_a: {
+        id: 'wall_a',
+        type: 'wall',
+        start: [0, 0],
+        end: [1, 0],
+        children: [null, 'item_x'],
+      },
+      item_x: { id: 'item_x', type: 'item' },
+    })
+    expect(strippedChildRefs).toBe(1)
+    expect((nodes.wall_a as { children: string[] }).children).toEqual(['item_x'])
+  })
+
+  test('drops childless zero-length walls and removes their parent reference', () => {
+    const { nodes, droppedWallIds } = healSceneNodes({
+      level_0: { id: 'level_0', type: 'level', children: ['wall_zero', 'wall_real'] },
+      wall_zero: { id: 'wall_zero', type: 'wall', start: [5, 5], end: [5, 5], children: [] },
+      wall_real: { id: 'wall_real', type: 'wall', start: [0, 0], end: [3, 0], children: [] },
+    })
+    expect(droppedWallIds).toEqual(['wall_zero'])
+    expect('wall_zero' in nodes).toBe(false)
+    expect((nodes.level_0 as { children: string[] }).children).toEqual(['wall_real'])
+  })
+
+  test('keeps a zero-length wall that still hosts a door/window', () => {
+    const { nodes, droppedWallIds } = healSceneNodes({
+      wall_z: { id: 'wall_z', type: 'wall', start: [1, 1], end: [1, 1], children: ['door_1'] },
+      door_1: { id: 'door_1', type: 'door' },
+    })
+    expect(droppedWallIds).toEqual([])
+    expect('wall_z' in nodes).toBe(true)
+  })
+
+  test('passes a clean scene through untouched', () => {
+    const input = {
+      wall_a: { id: 'wall_a', type: 'wall', start: [0, 0], end: [2, 0], children: ['door_1'] },
+      door_1: { id: 'door_1', type: 'door' },
+    }
+    const { nodes, droppedWallIds, strippedChildRefs } = healSceneNodes(input)
+    expect(droppedWallIds).toEqual([])
+    expect(strippedChildRefs).toBe(0)
+    expect(nodes.wall_a).toBe(input.wall_a)
+  })
+})

packages/core/src/utils/heal-scene-graph.ts

@@ -0,0 +1,83 @@
+// Repairs scene-graph corruption that pre-dates the source fixes, so existing
+// saved scenes still load. Two known kinds of damage, both produced by the
+// capture wall-merge before it was fixed:
+//
+//  1. A `children` array containing a non-string entry. The merge re-attached a
+//     wall-hosted item without minting an id, so `undefined` was pushed into the
+//     wall's children — which serializes to `[null]`. The wall schema rejects
+//     `null` children, so the whole scene fails to load.
+//  2. A zero-length wall (start === end). It renders nothing, but lingers as a
+//     junk node and is a foot-gun for snapping/mitering.
+//
+// Both are also prevented at the source now (see merge-walls.ts and the wall
+// miter limit); this is the load-time safety net for already-saved scenes.
+
+const ZERO_LENGTH_EPS = 1e-6
+
+export interface HealSceneResult {
+  nodes: Record<string, unknown>
+  /** Ids of zero-length walls that were dropped. */
+  droppedWallIds: string[]
+  /** Count of non-string (e.g. null) entries removed from `children` arrays. */
+  strippedChildRefs: number
+}
+
+function isWallLike(node: unknown): node is { start: [number, number]; end: [number, number] } {
+  if (!node || typeof node !== 'object') return false
+  const n = node as Record<string, unknown>
+  return (
+    n.type === 'wall' &&
+    Array.isArray(n.start) &&
+    Array.isArray(n.end) &&
+    typeof n.start[0] === 'number' &&
+    typeof n.start[1] === 'number' &&
+    typeof n.end[0] === 'number' &&
+    typeof n.end[1] === 'number'
+  )
+}
+
+/**
+ * Returns a healed copy of a `nodes` map. Pure — does not mutate `input`.
+ * Nodes that need no repair are passed through by reference.
+ */
+export function healSceneNodes(input: Record<string, unknown>): HealSceneResult {
+  const droppedWallIds: string[] = []
+
+  // Pass 1: drop childless zero-length walls. (Only childless ones — a wall
+  // carrying a door/window must keep its hosts, degenerate or not.)
+  const kept: Record<string, unknown> = {}
+  for (const [id, node] of Object.entries(input)) {
+    if (isWallLike(node)) {
+      const children = (node as { children?: unknown }).children
+      const childless = !Array.isArray(children) || children.length === 0
+      const dx = node.end[0] - node.start[0]
+      const dz = node.end[1] - node.start[1]
+      if (childless && Math.hypot(dx, dz) <= ZERO_LENGTH_EPS) {
+        droppedWallIds.push(id)
+        continue
+      }
+    }
+    kept[id] = node
+  }
+
+  const dropped = new Set(droppedWallIds)
+  let strippedChildRefs = 0
+
+  // Pass 2: clean `children` arrays — drop non-string entries (the `[null]` bug)
+  // and references to walls we just removed.
+  const nodes: Record<string, unknown> = {}
+  for (const [id, node] of Object.entries(kept)) {
+    const children = (node as { children?: unknown })?.children
+    if (Array.isArray(children)) {
+      const cleaned = children.filter((c): c is string => typeof c === 'string' && !dropped.has(c))
+      if (cleaned.length !== children.length) {
+        strippedChildRefs += children.length - cleaned.length
+        nodes[id] = { ...(node as Record<string, unknown>), children: cleaned }
+        continue
+      }
+    }
+    nodes[id] = node
+  }
+
+  return { nodes, droppedWallIds, strippedChildRefs }
+}

packages/core/src/validation/validate-build-json.ts

@@ -1,4 +1,5 @@
 import { AnyNode, type AnyNodeType } from '../schema/types'
+import { healSceneNodes } from '../utils/heal-scene-graph'
 
 export type ValidationSeverity = 'error' | 'warning'
 
@@ -127,9 +128,22 @@ export function validateBuildJson(input: unknown): ValidateBuildJsonResult {
     }
   }
 
-  const nodes = nodesRaw as Record<string, unknown>
+  // Heal known pre-existing corruption (null children, zero-length walls) up
+  // front, so a scene saved before the source fixes still imports instead of
+  // hard-failing schema validation. `parsed` below carries the repaired nodes.
+  const { nodes, droppedWallIds, strippedChildRefs } = healSceneNodes(
+    nodesRaw as Record<string, unknown>,
+  )
   const rootNodeIds = rootNodeIdsRaw as string[]
 
+  if (strippedChildRefs > 0 || droppedWallIds.length > 0) {
+    warnings.push({
+      severity: 'warning',
+      code: 'auto_repaired',
+      message: `Repaired on import: removed ${strippedChildRefs} invalid child reference${strippedChildRefs === 1 ? '' : 's'} and ${droppedWallIds.length} zero-length wall${droppedWallIds.length === 1 ? '' : 's'}.`,
+    })
+  }
+
   if (rootNodeIds.length === 0) {
     errors.push({
       severity: 'error',

packages/nodes/src/ceiling/move-tool.tsx

@@ -9,6 +9,7 @@ import {
   polygonAnchors,
   resolveAlignment,
   sceneRegistry,
+  snapScalar,
   useLiveTransforms,
   useScene,
 } from '@pascal-app/core'
@@ -36,10 +37,10 @@ import { BufferGeometry, DoubleSide, Path, Shape, ShapeGeometry, Vector3 } from
  * mesh's X/Z position on rebuild (`mesh.position.x = 0`,
  * `mesh.position.z = 0`) so the visual transitions smoothly.
  *
- * 0.5m grid snap (matches legacy).
+ * Snaps to the editor's configured grid step (Shift bypasses).
  */
 function snap(value: number) {
-  return Math.round(value * 2) / 2
+  return snapScalar(value, useEditor.getState().gridSnapStep)
 }
 
 /** Figma-style alignment-snap threshold (meters), matching the other tools. */

packages/nodes/src/ceiling/tool.tsx

@@ -6,6 +6,7 @@ import {
   type GridEvent,
   type LevelNode,
   snapPointAlongAngleRay,
+  snapPointToGrid,
   useScene,
 } from '@pascal-app/core'
 import {
@@ -93,9 +94,9 @@ export const CeilingTool: React.FC = () => {
       if (!(cursorRef.current && gridCursorRef.current)) return
       const rawPoint: [number, number] = [event.localPosition[0], event.localPosition[2]]
       const bypassSnap = shiftPressed.current || event.nativeEvent?.shiftKey === true
-      const gridX = Math.round(rawPoint[0] * 2) / 2
+      const gridPosition: [number, number] = bypassSnap
-      const gridZ = Math.round(rawPoint[1] * 2) / 2
+        ? rawPoint
-      const gridPosition: [number, number] = bypassSnap ? rawPoint : [gridX, gridZ]
+        : [...snapPointToGrid(rawPoint, useEditor.getState().gridSnapStep)]
       setCursorPosition(gridPosition)
       setLevelY(event.localPosition[1])
       const ceilingY = event.localPosition[1] + CEILING_HEIGHT

packages/nodes/src/slab/tool.tsx

@@ -6,6 +6,7 @@ import {
   type GridEvent,
   type LevelNode,
   snapPointAlongAngleRay,
+  snapPointToGrid,
   useScene,
 } from '@pascal-app/core'
 import {
@@ -76,9 +77,9 @@ export const SlabTool: React.FC = () => {
       if (!cursorRef.current) return
       const rawPoint: [number, number] = [event.localPosition[0], event.localPosition[2]]
       const bypassSnap = shiftPressed.current || event.nativeEvent?.shiftKey === true
-      const gridX = Math.round(rawPoint[0] * 2) / 2
+      const gridPosition: [number, number] = bypassSnap
-      const gridZ = Math.round(rawPoint[1] * 2) / 2
+        ? rawPoint
-      const gridPosition: [number, number] = bypassSnap ? rawPoint : [gridX, gridZ]
+        : [...snapPointToGrid(rawPoint, useEditor.getState().gridSnapStep)]
       setCursorPosition(gridPosition)
       setLevelY(event.localPosition[1])
       const lastPoint = points[points.length - 1]

packages/nodes/src/spawn/tool.tsx

@@ -1,6 +1,13 @@
 'use client'
 
-import { emitter, type GridEvent, SpawnNode, sceneRegistry, useScene } from '@pascal-app/core'
+import {
+  emitter,
+  type GridEvent,
+  SpawnNode,
+  sceneRegistry,
+  snapScalar,
+  useScene,
+} from '@pascal-app/core'
 import {
   CursorSphere,
   getFloorStackPreviewPosition,
@@ -11,7 +18,7 @@ import { useViewer } from '@pascal-app/viewer'
 import { useEffect, useRef } from 'react'
 import { type Group, Vector3 } from 'three'
 
-const roundToHalf = (value: number) => Math.round(value * 2) / 2
+const snapToGrid = (value: number) => snapScalar(value, useEditor.getState().gridSnapStep)
 const worldVector = new Vector3()
 
 function getExistingSpawnIds() {
@@ -31,14 +38,14 @@ function getLevelLocalPosition(
   if (!levelObject) {
     return bypassSnap
       ? [event.localPosition[0], 0, event.localPosition[2]]
-      : [roundToHalf(event.localPosition[0]), 0, roundToHalf(event.localPosition[2])]
+      : [snapToGrid(event.localPosition[0]), 0, snapToGrid(event.localPosition[2])]
   }
   worldVector.set(event.position[0], event.position[1], event.position[2])
   levelObject.updateWorldMatrix(true, false)
   levelObject.worldToLocal(worldVector)
   return bypassSnap
     ? [worldVector.x, 0, worldVector.z]
-    : [roundToHalf(worldVector.x), 0, roundToHalf(worldVector.z)]
+    : [snapToGrid(worldVector.x), 0, snapToGrid(worldVector.z)]
 }
 
 /**
@@ -58,11 +65,11 @@ const SpawnTool = () => {
 
     const onGridMove = (event: GridEvent) => {
       // Cursor lives in the ToolManager's building-local group. Use
-      // event.localPosition directly (already building-local) with the
+      // event.localPosition directly (already building-local), snapped to the
-      // same half-meter snap the legacy tool uses.
+      // editor's configured grid step (Shift bypasses).
       const bypassSnap = event.nativeEvent?.shiftKey === true
-      const nextX = bypassSnap ? event.localPosition[0] : roundToHalf(event.localPosition[0])
+      const nextX = bypassSnap ? event.localPosition[0] : snapToGrid(event.localPosition[0])
-      const nextZ = bypassSnap ? event.localPosition[2] : roundToHalf(event.localPosition[2])
+      const nextZ = bypassSnap ? event.localPosition[2] : snapToGrid(event.localPosition[2])
       const position: [number, number, number] = [nextX, 0, nextZ]
       const previewNode = SpawnNode.parse({
         name: 'Spawn Point',