fix(sorter): use model-space index when filtered children precede drop target

getChildrenDim skips children whose DOM element does not match the itemSel
(e.g. comment nodes rendered as nodeType=8). findPosition returns an index
into the filtered dims array, which was used directly as the model-space
insert index — off by however many children were skipped earlier in the list.

Fix: stamp each Dimension with its unfiltered position (indexEl) in
getChildrenDim, then translate in getDropPosition via
`(dimension.indexEl ?? dimensionIndex)`.

Also guards getDropPosition against the crash that occurred when all
children were filtered (childrenDimensions.length === 0) but the parent
still had model children.

Fixes drops being off-by-N when MSO conditional comment siblings exist
(e.g. content produced by grapesjs-mjml).

packages/core/src/utils/sorter/Dimension.ts

@@ -12,6 +12,8 @@ export default class Dimension {
   public width: number;
   public offsets: ReturnType<CanvasModule['getElementOffsets']>;
   public dir?: boolean;
+  /** Index of this child in the *unfiltered* children list (model-space). Set by getChildrenDim. */
+  public indexEl?: number;
 
   /**
    * Initializes the DimensionCalculator with the given initial dimensions.
@@ -34,6 +36,7 @@ export default class Dimension {
     this.width = initialDimensions.width;
     this.offsets = initialDimensions.offsets;
     this.dir = initialDimensions.dir;
+    this.indexEl = initialDimensions.indexEl;
   }
 
   /**
@@ -111,6 +114,7 @@ export default class Dimension {
       width: this.width,
       offsets: { ...this.offsets }, // Shallow copy of offsets
       dir: this.dir,
+      indexEl: this.indexEl,
     });
   }
 

packages/core/src/utils/sorter/DropLocationDeterminer.ts

@@ -252,10 +252,14 @@ export class DropLocationDeterminer<T, NodeType extends SortableTreeNode<T>> ext
     let placeholderDimensions = nodeDimensions.clone(),
       index = 0,
       placement = 'inside' as Placement;
-    if (nodeHasChildren) {
+    if (nodeHasChildren && childrenDimensions.length > 0) {
-      ({ index, placement } = findPosition(childrenDimensions, mouseX, mouseY));
+      const { index: dimensionIndex, placement: foundPlacement } = findPosition(childrenDimensions, mouseX, mouseY);
-      placeholderDimensions = childrenDimensions[index].clone();
+      placement = foundPlacement;
-      index = index + (placement == 'after' ? 1 : 0);
+      const dimension = childrenDimensions[dimensionIndex];
+      placeholderDimensions = dimension.clone();
+      // Use the unfiltered (model-space) index so skipped children (e.g. comment
+      // nodes) don't cause an off-by-N insertion error.
+      index = (dimension.indexEl ?? dimensionIndex) + (placement == 'after' ? 1 : 0);
     }
 
     return {
@@ -492,6 +496,7 @@ export class DropLocationDeterminer<T, NodeType extends SortableTreeNode<T>> ext
 
       const dim = this.getDim(el);
       dim.dir = this.getDirection(el, targetElement);
+      dim.indexEl = i;
       dims.push(dim);
     });
 

packages/core/test/specs/utils/sorter/sorterDropIndex.ts

@@ -0,0 +1,230 @@
+import { DropLocationDeterminer } from '../../../../src/utils/sorter/DropLocationDeterminer';
+import { SortableTreeNode } from '../../../../src/utils/sorter/SortableTreeNode';
+import Dimension from '../../../../src/utils/sorter/Dimension';
+import { DragDirection, DragSource } from '../../../../src/utils/sorter/types';
+import { setupTestEditor } from '../../../common';
+import Editor from '../../../../src/editor';
+
+// Minimal concrete SortableTreeNode for test use only.
+// The treeClass constructor signature requires (model, dragSource?); we stash
+// the element and children on the instance after construction via factory helpers.
+class TestNode extends SortableTreeNode<any> {
+  public _element: HTMLElement | undefined;
+  public _children: TestNode[] = [];
+  public _parent: TestNode | null = null;
+
+  constructor(model: any = {}, dragSource: DragSource<any> = {}) {
+    super(model, dragSource);
+  }
+
+  static create(element?: HTMLElement, children: TestNode[] = []): TestNode {
+    const node = new TestNode();
+    node._element = element;
+    node._children = children;
+    node._children.forEach((c) => (c._parent = node));
+    return node;
+  }
+
+  get element() {
+    return this._element;
+  }
+  get view() {
+    return undefined;
+  }
+  getChildren() {
+    return this._children.length ? this._children : null;
+  }
+  getParent() {
+    return this._parent;
+  }
+  addChildAt(node: TestNode, index: number) {
+    return node;
+  }
+  removeChildAt(_index: number) {}
+  indexOfChild(node: TestNode) {
+    return this._children.indexOf(node);
+  }
+  canMove() {
+    return true;
+  }
+  equals(other?: TestNode): other is TestNode {
+    return other === this;
+  }
+}
+
+function makeDeterminer(em: any, itemSel = '.gjs-comp') {
+  const container = document.createElement('div');
+  return new DropLocationDeterminer<any, TestNode>({
+    em,
+    treeClass: TestNode,
+    containerContext: {
+      container,
+      itemSel,
+      document,
+    },
+    positionOptions: { canvasRelative: false, relative: false, windowMargin: 0 },
+    dragDirection: DragDirection.Vertical,
+    eventHandlers: {},
+  });
+}
+
+function makeDim(top: number, height: number, indexEl?: number): Dimension {
+  const d = new Dimension({ top, left: 0, height, width: 200, offsets: {} as any, dir: true });
+  d.indexEl = indexEl;
+  return d;
+}
+
+describe('Sorter drop-index offset (comment-node skew)', () => {
+  let editor: Editor;
+  let fixtures: HTMLElement;
+
+  beforeEach(() => {
+    ({ editor, fixtures } = setupTestEditor({ withCanvas: true }));
+  });
+
+  afterEach(() => {
+    jest.restoreAllMocks();
+    editor.destroy();
+  });
+
+  describe('Dimension', () => {
+    test('clone preserves indexEl', () => {
+      const d = makeDim(0, 100, 3);
+      const cloned = d.clone();
+      expect(cloned.indexEl).toBe(3);
+    });
+
+    test('clone without indexEl keeps it undefined', () => {
+      const d = makeDim(0, 100);
+      expect(d.clone().indexEl).toBeUndefined();
+    });
+  });
+
+  describe('getChildrenDim', () => {
+    test('stamps indexEl with unfiltered child position, skipping null-element children', () => {
+      const determiner = makeDeterminer(editor.getModel());
+      const parentEl = document.createElement('div');
+
+      // Children: [no-el, div, no-el, div, div] — indices 0,1,2,3,4 in model
+      const div1 = document.createElement('div');
+      const div2 = document.createElement('div');
+      const div3 = document.createElement('div');
+      div1.className = 'gjs-comp';
+      div2.className = 'gjs-comp';
+      div3.className = 'gjs-comp';
+
+      const parent = TestNode.create(parentEl, [
+        TestNode.create(undefined),
+        TestNode.create(div1),
+        TestNode.create(undefined),
+        TestNode.create(div2),
+        TestNode.create(div3),
+      ]);
+
+      jest
+        .spyOn(determiner as any, 'getDim')
+        .mockImplementation((el: unknown) => makeDim(el === div1 ? 0 : el === div2 ? 100 : 200, 100));
+      jest.spyOn(determiner as any, 'getDirection').mockReturnValue(true);
+
+      const dims: Dimension[] = (determiner as any).getChildrenDim(parent);
+
+      expect(dims).toHaveLength(3);
+      expect(dims[0].indexEl).toBe(1);
+      expect(dims[1].indexEl).toBe(3);
+      expect(dims[2].indexEl).toBe(4);
+    });
+  });
+
+  describe('getDropPosition', () => {
+    test('returns model-space index when earlier children are filtered out', () => {
+      // Scenario: parent [skip, div, skip, div, div]
+      // Mouse after the last div → should land at model index 5 (after index 4)
+      const determiner = makeDeterminer(editor.getModel());
+      const parentEl = document.createElement('div');
+
+      const parent = TestNode.create(parentEl, [
+        TestNode.create(undefined),
+        TestNode.create(document.createElement('div')),
+        TestNode.create(undefined),
+        TestNode.create(document.createElement('div')),
+        TestNode.create(document.createElement('div')),
+      ]);
+
+      const parentDim = makeDim(0, 300);
+      parent.nodeDimensions = parentDim;
+      // Pre-supply childrenDimensions with correct indexEl so getChildrenDim is bypassed
+      parent.childrenDimensions = [makeDim(0, 100, 1), makeDim(100, 100, 3), makeDim(200, 100, 4)];
+
+      // y=280 is inside the last child (top=200, height=100) and past its center
+      const result = (determiner as any).getDropPosition(parent, 100, 280);
+
+      expect(result.placement).toBe('after');
+      // Without fix: index would be 2+1=3; with fix: (4 ?? 2)+1=5
+      expect(result.index).toBe(5);
+    });
+
+    test('returns correct index when drop is before a middle child', () => {
+      const determiner = makeDeterminer(editor.getModel());
+      const parentEl = document.createElement('div');
+
+      const parent = TestNode.create(parentEl, [
+        TestNode.create(undefined),
+        TestNode.create(document.createElement('div')),
+        TestNode.create(undefined),
+        TestNode.create(document.createElement('div')),
+        TestNode.create(document.createElement('div')),
+      ]);
+
+      parent.nodeDimensions = makeDim(0, 300);
+      // Pre-supply childrenDimensions: skip items 0 and 2
+      parent.childrenDimensions = [makeDim(0, 100, 1), makeDim(100, 100, 3), makeDim(200, 100, 4)];
+
+      // y=40 is in first child (top=0, height=100) before its center (50)
+      const result = (determiner as any).getDropPosition(parent, 100, 40);
+
+      expect(result.placement).toBe('before');
+      // dimensionIndex=0, indexEl=1, placement='before' → index = 1+0 = 1
+      expect(result.index).toBe(1);
+    });
+
+    test('does not crash when all children lack measurable elements', () => {
+      // Second bug: parent with only un-measurable children → childrenDimensions=[]
+      const determiner = makeDeterminer(editor.getModel());
+      const parentEl = document.createElement('div');
+
+      const parent = TestNode.create(parentEl, [TestNode.create(undefined), TestNode.create(undefined)]);
+
+      parent.nodeDimensions = makeDim(0, 200);
+      parent.childrenDimensions = []; // all filtered out
+
+      // Should not throw and should return index=0, placement='inside'
+      let result: any;
+      expect(() => {
+        result = (determiner as any).getDropPosition(parent, 100, 100);
+      }).not.toThrow();
+
+      expect(result.index).toBe(0);
+      expect(result.placement).toBe('inside');
+    });
+
+    test('clean children (no skipped nodes) are unaffected', () => {
+      // Regression: if no children are filtered, indexEl == dimensionIndex and result is identical
+      const determiner = makeDeterminer(editor.getModel());
+      const parentEl = document.createElement('div');
+
+      const parent = TestNode.create(parentEl, [
+        TestNode.create(document.createElement('div')),
+        TestNode.create(document.createElement('div')),
+        TestNode.create(document.createElement('div')),
+      ]);
+
+      parent.nodeDimensions = makeDim(0, 300);
+      parent.childrenDimensions = [makeDim(0, 100, 0), makeDim(100, 100, 1), makeDim(200, 100, 2)];
+
+      // y=280 → after last child (index 2) → 2+1=3
+      const result = (determiner as any).getDropPosition(parent, 100, 280);
+      expect(result.placement).toBe('after');
+      expect(result.index).toBe(3);
+    });
+  });
+});