Fix memory leak in plugin tree views (#12353)

Fixes a bug in `TreeViewExtImpl<T>.getChildren(parentId: string)`,
where newly created top-level children were added to the old root node
instead of the new one. This caused a memory leak by retaining an implicit
reference to the old root node from the new one and failing to properly track
and, hence, dispose the top-level children.

The fix ensures that old nodes get disposed and become subject to garbage
collection by reusing the single root node instead of creating a new one
on full refresh.

See #10404 (comment)
for more information.

Fixes #10404.

packages/plugin-ext/src/plugin/tree/tree-views.ts

@@ -389,7 +389,7 @@ class TreeViewExtImpl<T> implements Disposable {
     }
 
     async getChildren(parentId: string): Promise<TreeViewItem[] | undefined> {
-        const parentNode = this.nodes.get(parentId);
+        let parentNode = this.nodes.get(parentId);
         const parent = parentNode?.value;
         if (parentId && !parent) {
             console.error(`No tree item with id '${parentId}' found.`);
@@ -398,9 +398,10 @@ class TreeViewExtImpl<T> implements Disposable {
         this.clearChildren(parentNode);
 
         // place root in the cache
-        if (parentId === '') {
+        if (parentId === '' && !parentNode) {
             const rootNodeDisposables = new DisposableCollection();
-            this.nodes.set(parentId, { id: '', disposables: rootNodeDisposables, dispose: () => { rootNodeDisposables.dispose(); } });
+            parentNode = { id: '', disposables: rootNodeDisposables, dispose: () => { rootNodeDisposables.dispose(); } };
+            this.nodes.set(parentId, parentNode);
         }
         // ask data provider for children for cached element
         const result = await this.options.treeDataProvider.getChildren(parent);