Create Async Subtree Loading for Neo.data.TreeStore

Goal

Enhance the Neo.data.TreeStore to support asynchronous loading of child nodes when a collapsed node is expanded, laying the groundwork for handling massive hierarchical datasets without a full BufferedStore.

Architecture & Mechanics

We will leverage the existing Store.load() mechanics (specifically the opts.append pattern) and Collection.splice() to inject new data into the tree without destroying the current view.

1. Pre-condition Enforcement:
   • If a node is loaded with expanded: true (or collapsed: false), the payload must contain its direct children. The TreeStore parsing logic should throw a warning if this condition is violated to prevent rendering empty folders.
2. Count Separation:
   • totalCount (Data Perspective): Represents the total size of the fully expanded tree on the backend (useful for metadata/pagination, but not for the scrollbar).
   • count (UI Perspective): Managed by the Collection. Represents the length of the flattened _items array (only the visible nodes). This drives the GridBody scrollbar height and aria-rowcount.
3. The Async Expand Flow: When expand(nodeId) is called on a node that does not have its children loaded in the #childrenMap:
   • Set the node's state to isLoading: true (triggers UI spinner).
   • Trigger a backend request for the subtree (e.g., store.load({ params: { parentId: nodeId }, append: true })).
   • Upon data arrival, parse the new subtree and update the internal #childrenMap.
   • Update the node's state: isLoading: false, expanded: true.
   • Execute a synchronous splice to inject the new child nodes into the flattened items array directly below the parent.
   • The resulting mutate event will automatically update the grid's rendering and scrollbar.

Future-Proofing

By relying on Collection.splice and the flattened _items array, this implementation provides the exact hooks needed for a future BufferedStore (which would inject placeholder rows instead of actual data rows, triggering targeted fetches as they scroll into view).

Dependencies

• Depends on the core TreeStore implementation (#9406).