fix(sorter): hit-test against the container's root node so sorting works in a shadow root

DropLocationDeterminer#getMouseTargetElement resolved the drop target with
`containerContext.document.elementFromPoint()`. `Document.elementFromPoint` does
not pierce shadow boundaries, so when a sorter container lives inside a shadow
root it returned the shadow host instead of the hovered item. The subsequent
`getFirstElementWithAModel` walk is bounded by
`containerContext.container.contains(el)`, which is false for the host, so it
returned null and handleMove bailed out on every sample: no placeholder, no
drop, silent failure.

This hits the Layer Manager when rendered into the host page via
`layerManager.appendTo` (ItemsView hardcodes `document: document`), making layer
reordering a no-op. The canvas is unaffected because its containerContext
document is the iframe document.

Fix: run the hit-test on `container.getRootNode()`, which is the ShadowRoot when
the container is in a shadow tree and the Document otherwise. `ShadowRoot`
implements the same DocumentOrShadowRoot mixin and resolves within its own tree,
so behaviour outside shadow DOM is unchanged; the isFunction guard falls back to
containerContext.document for a detached container.

Introduced by the sorter refactor in #6149 (0.22.1), which replaced the previous
`e.target` lookup — not retargeted for a listener inside the same shadow tree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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

@@ -4,7 +4,7 @@ import EditorModel from '../../editor/model/Editor';
 import { isTextNode, off, on } from '../dom';
 import { SortableTreeNode } from './SortableTreeNode';
 import { Placement, PositionOptions, DragDirection, SorterEventHandlers, CustomTarget, DragSource } from './types';
-import { bindAll, each } from 'underscore';
+import { bindAll, each, isFunction } from 'underscore';
 import { matches, findPosition, offset, isStyleInFlow } from './SorterUtils';
 import { RateLimiter } from './RateLimiter';
 import Dimension from './Dimension';
@@ -287,9 +287,24 @@ export class DropLocationDeterminer<T, NodeType extends SortableTreeNode<T>> ext
     return newHoveredNode;
   }
 
+  /**
+   * Resolves the root to run the hit-test against.
+   *
+   * `Document.elementFromPoint` doesn't pierce shadow boundaries, so when the container lives
+   * inside a shadow root it would always return the host element instead of the hovered item.
+   * `ShadowRoot` implements the same `DocumentOrShadowRoot` mixin and resolves within its own tree.
+   */
+  private getContainerContextRoot(): DocumentOrShadowRoot {
+    const root = this.containerContext.container?.getRootNode() as Partial<DocumentOrShadowRoot> | undefined;
+    if (isFunction(root?.elementFromPoint)) {
+      return root as DocumentOrShadowRoot;
+    }
+    return this.containerContext.document;
+  }
+
   private getMouseTargetElement(mouseEvent: MouseEvent) {
     const customTarget = this.containerContext.customTarget;
-    let mouseTarget = this.containerContext.document.elementFromPoint(
+    let mouseTarget = this.getContainerContextRoot().elementFromPoint(
       mouseEvent.clientX,
       mouseEvent.clientY,
     ) as HTMLElement;