fix(editor): harden snapping edge cases

- Preserve rounded auto-layout preview displacement through drop commits
- Bound imported layout-guide counts and tiny grid steps
- Normalize missing guide arrays in direct graph hydration paths
- Add regression coverage for auto-layout movement and guide limits

packages/core/src/kiwi/fig/parse/transfer.ts

@@ -115,7 +115,9 @@ export function cloneSceneGraphForFigExport(graph: SceneGraph): SceneGraph {
 export function deserializeSceneGraph(data: SerializedSceneGraph): SceneGraph {
   const graph = new SceneGraph()
   graph.rootId = data.rootId
-  graph.nodes = new Map(data.nodes)
+  graph.nodes = new Map(
+    data.nodes.map(([id, node]) => [id, { ...node, guides: node.guides ?? [] }])
+  )
   graph.images = new Map(data.images)
   graph.variables = new Map(data.variables)
   graph.variableCollections = new Map(data.variableCollections)

packages/core/src/library/serialization.ts

@@ -39,7 +39,13 @@ export function deserializeLibraryRevision(
 ): ComponentLibraryRevision {
   const graph = new SceneGraph()
   graph.rootId = revision.graph.rootId
-  graph.nodes = new Map(revision.graph.nodes.map(([id, node]) => [id, structuredClone(node)]))
+  graph.nodes = new Map(
+    revision.graph.nodes.map(([id, node]) => {
+      const cloned = structuredClone(node)
+      cloned.guides ??= []
+      return [id, cloned]
+    })
+  )
   graph.images = new Map(
     revision.graph.images.map(([hash, bytes]) => [hash, new Uint8Array(bytes)])
   )

packages/scene-graph/src/layout-guides.ts

@@ -5,6 +5,14 @@ export interface LayoutGuideLine {
   position: number
 }
 
+const MAX_LAYOUT_GUIDE_LINES = 10_000
+
+function normalizedCount(grid: LayoutGrid): number {
+  const value = grid.count ?? grid.numSections ?? 1
+  if (!Number.isFinite(value)) return 0
+  return Math.min(MAX_LAYOUT_GUIDE_LINES, Math.max(0, Math.floor(value)))
+}
+
 function pattern(grid: LayoutGrid): 'COLUMNS' | 'ROWS' | 'GRID' {
   if (grid.pattern === 'GRID' || grid.pattern === 'ROWS') return grid.pattern
   if (grid.axis === 'Y') return 'ROWS'
@@ -20,14 +28,14 @@ function alignment(grid: LayoutGrid): 'MIN' | 'CENTER' | 'MAX' | 'STRETCH' {
 function sectionSize(nodeSize: number, grid: LayoutGrid): number {
   const fixed = grid.sectionSize ?? 0
   if (alignment(grid) !== 'STRETCH') return fixed
-  const count = grid.count ?? grid.numSections ?? 1
+  const count = normalizedCount(grid)
   const gutter = grid.gutterSize ?? 0
   const offset = grid.offset ?? 0
   return (nodeSize - offset * 2 - Math.max(0, count - 1) * gutter) / count
 }
 
 function start(nodeSize: number, grid: LayoutGrid, size: number): number {
-  const count = grid.count ?? grid.numSections ?? 1
+  const count = normalizedCount(grid)
   const gutter = grid.gutterSize ?? 0
   const offset = grid.offset ?? 0
   const span = count * size + Math.max(0, count - 1) * gutter
@@ -50,8 +58,8 @@ export function layoutGuideSections(
   const axis = pattern(grid) === 'ROWS' ? 'y' : 'x'
   const nodeSize = axis === 'x' ? node.width : node.height
   const size = sectionSize(nodeSize, grid)
-  const count = grid.count ?? grid.numSections ?? 1
-  if (!Number.isFinite(count) || count <= 0 || size <= 0) return []
+  const count = normalizedCount(grid)
+  if (count <= 0 || size <= 0) return []
   const first = start(nodeSize, grid, size)
   const step = size + (grid.gutterSize ?? 0)
   return Array.from({ length: count }, (_, index) => {
@@ -67,10 +75,18 @@ export function layoutGuideLines(node: Pick<SceneNode, 'width' | 'height'>, grid
   if (gridPattern === 'GRID') {
     const size = grid.sectionSize ?? 0
     if (size <= 0) return lines
-    for (let x = grid.offset ?? 0; x <= node.width; x += size) {
+    for (
+      let index = 0, x = grid.offset ?? 0;
+      x <= node.width && index < MAX_LAYOUT_GUIDE_LINES;
+      index++, x += size
+    ) {
       lines.push({ axis: 'x', position: x })
     }
-    for (let y = grid.offset ?? 0; y <= node.height; y += size) {
+    for (
+      let index = 0, y = grid.offset ?? 0;
+      y <= node.height && index < MAX_LAYOUT_GUIDE_LINES;
+      index++, y += size
+    ) {
       lines.push({ axis: 'y', position: y })
     }
     return lines

packages/vue/src/shared/input/move.ts

@@ -86,8 +86,16 @@ export function handleMoveMove(
   if (dropParent && dropParent.layoutMode !== 'NONE') {
     computeAutoLayoutIndicatorForFrame(dropParent, cx, cy, editor)
     editor.setDropTarget(dropParent.id)
+    let firstApplied: { dx: number; dy: number } | null = null
     for (const [id, orig] of d.originals) {
-      editor.graph.updateNodePositionPreview(id, Math.round(orig.x + dx), Math.round(orig.y + dy))
+      const previewX = Math.round(orig.x + dx)
+      const previewY = Math.round(orig.y + dy)
+      firstApplied ??= { dx: previewX - orig.x, dy: previewY - orig.y }
+      editor.graph.updateNodePositionPreview(id, previewX, previewY)
+    }
+    if (firstApplied) {
+      d.appliedDx = firstApplied.dx
+      d.appliedDy = firstApplied.dy
     }
     editor.requestRepaint()
     return

tests/engine/scene-graph/layout-guides.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, test } from 'bun:test'
+
+import { layoutGuideLines, layoutGuideSections } from '@open-pencil/scene-graph/layout-guides'
+
+describe('layout guide geometry bounds', () => {
+  test('normalizes fractional section counts', () => {
+    expect(
+      layoutGuideSections(
+        { width: 100, height: 100 },
+        { pattern: 'COLUMNS', count: 2.8, sectionSize: 10 }
+      )
+    ).toHaveLength(2)
+  })
+
+  test('caps pathological section and grid line counts', () => {
+    expect(
+      layoutGuideSections(
+        { width: 100, height: 100 },
+        { pattern: 'COLUMNS', count: Number.MAX_SAFE_INTEGER, sectionSize: 0.001 }
+      )
+    ).toHaveLength(10_000)
+    expect(
+      layoutGuideLines(
+        { width: 100, height: 100 },
+        { pattern: 'GRID', sectionSize: Number.MIN_VALUE }
+      )
+    ).toHaveLength(20_000)
+  })
+})

tests/engine/vue/input/move-threshold.test.ts

@@ -138,6 +138,33 @@ describe('selection move drag threshold', () => {
     expect(drag.appliedDy).toBeCloseTo(74 * Math.sin(angle) + 43 * Math.cos(angle), 3)
   })
 
+  test('stores rounded preview displacement when targeting auto-layout', () => {
+    const editor = createEditor()
+    const pageId = editor.state.currentPageId
+    const moving = editor.graph.createNode('RECTANGLE', pageId, {
+      x: 10.25,
+      y: 20.25,
+      width: 20,
+      height: 20
+    })
+    editor.graph.createNode('FRAME', pageId, {
+      x: 100,
+      y: 100,
+      width: 200,
+      height: 100,
+      layoutMode: 'HORIZONTAL'
+    })
+    editor.select([moving.id])
+    const drag = createSelectionMoveDrag(10.25, 20.25, 0, 0, editor, false)
+    if (drag.type !== 'move') throw new Error('Expected move drag')
+
+    handleMoveMove(drag, 150.6, 130.7, 10, 10, editor)
+
+    expect(drag.appliedDx).toBeCloseTo(140.75)
+    expect(drag.appliedDy).toBeCloseTo(110.75)
+    expect(editor.graph.getNode(moving.id)).toMatchObject({ x: 151, y: 131 })
+  })
+
   test('removes duplicate created for alt-click without movement', () => {
     const editor = createEditor()
     const pageId = editor.state.currentPageId