Tree: Fix incorrect error notification when deleting last child (closes #20977) (#20985)

* Fix infinite recursion and incorrect error notifications in tree children loading

This commit addresses two critical issues in the tree item children manager:

1. **Infinite recursion vulnerability**: The #resetChildren() method called
   loadChildren(), which could recursively call #resetChildren() again if
   the underlying issue persisted, creating an infinite loop.

2. **Inappropriate error messages**: The "Menu loading failed" notification
   was shown even in legitimate scenarios, such as when deleting the last
   child of a node, where an empty tree is the expected outcome.

Changes made:

- Add ResetReason type ('error' | 'empty' | 'fallback') to differentiate
  between error states and expected empty states

- Extract #loadChildrenWithOffsetPagination() as a terminal fallback method
  that uses only offset pagination and never calls #resetChildren(),
  structurally preventing recursion

- Update #resetChildren() to:
  - Accept a reason parameter to determine whether to show error notification
  - Reset all retry counters (#loadChildrenRetries, #loadPrevItemsRetries,
    #loadNextItemsRetries) to ensure clean state
  - Call #loadChildrenWithOffsetPagination() instead of loadChildren()
  - Only show error notification when reason is 'error'

- Update all call sites of #resetChildren() with appropriate reasons:
  - 'error' when retries are exhausted (actual failures)
  - 'empty' or 'fallback' when no new target is found (may be expected,
    e.g., after deleting items)

The fix makes infinite recursion structurally impossible by creating a
one-way flow: target-based loading can fall back to #resetChildren(),
which calls offset-only loading that never recurses back.

* Fix undefined items array causing tree to break after deletion

This fixes the root cause of issue #20977 where deleting a document type
would cause the tree to "forever load" with a JavaScript error.

The error occurred in #getTargetResultHasValidParents() which called .every()
on data without checking if it was undefined. When the API returned undefined
items (e.g., after deleting the last child), this caused:

TypeError: can't access property "every", e is undefined

The fix adds a guard to check if data is undefined before calling .every(),
returning false in that case to trigger the proper error handling flow.

* Address code review feedback on terminal fallback method

- Change error throwing to silent return for graceful failure handling
- Remove target pagination state updates from offset-only loading method
- Update JSDoc to clarify that method does not throw errors

src/Umbraco.Web.UI.Client/src/packages/core/tree/tree-item/tree-item-children.manager.ts

@@ -23,6 +23,8 @@ import {
 } from '@umbraco-cms/backoffice/entity-action';
 import { UMB_NOTIFICATION_CONTEXT } from '@umbraco-cms/backoffice/notification';
 
+type ResetReason = 'error' | 'empty' | 'fallback';
+
 export class UmbTreeItemChildrenManager<
 	TreeItemType extends UmbTreeItemModel = UmbTreeItemModel,
 	TreeRootType extends UmbTreeRootModel = UmbTreeRootModel,
@@ -218,7 +220,7 @@ export class UmbTreeItemChildrenManager<
 	async #loadChildren(reload = false) {
 		if (this.#loadChildrenRetries > this.#requestMaxRetries) {
 			this.#loadChildrenRetries = 0;
-			this.#resetChildren();
+			this.#resetChildren('error');
 			return;
 		}
 
@@ -302,7 +304,7 @@ export class UmbTreeItemChildrenManager<
 					We cancel the base target and load using skip/take pagination instead.
 					This can happen if deep linked to a non existing item or all retries have failed.
 				*/
-				this.#resetChildren();
+				this.#resetChildren(this.#children.getValue().length === 0 ? 'empty' : 'fallback');
 			}
 		}
 
@@ -329,7 +331,7 @@ export class UmbTreeItemChildrenManager<
 		if (this.#loadPrevItemsRetries > this.#requestMaxRetries) {
 			// If we have exceeded the maximum number of retries, we need to reset the base target and load from the top
 			this.#loadPrevItemsRetries = 0;
-			this.#resetChildren();
+			this.#resetChildren('error');
 			return;
 		}
 
@@ -378,7 +380,7 @@ export class UmbTreeItemChildrenManager<
 					If we can't find a new end target we reload the children from the top.
 					We cancel the base target and load using skip/take pagination instead.
 				*/
-				this.#resetChildren();
+				this.#resetChildren(this.#children.getValue().length === 0 ? 'empty' : 'fallback');
 			}
 		}
 
@@ -409,7 +411,7 @@ export class UmbTreeItemChildrenManager<
 		if (this.#loadNextItemsRetries > this.#requestMaxRetries) {
 			// If we have exceeded the maximum number of retries, we need to reset the base target and load from the top
 			this.#loadNextItemsRetries = 0;
-			this.#resetChildren();
+			this.#resetChildren('error');
 			return;
 		}
 
@@ -467,7 +469,7 @@ export class UmbTreeItemChildrenManager<
 					If we can't find a new end target we reload the children from the top.
 					We cancel the base target and load using skip/take pagination instead.
 				*/
-				this.#resetChildren();
+				this.#resetChildren(this.#children.getValue().length === 0 ? 'empty' : 'fallback');
 			}
 		}
 
@@ -520,12 +522,81 @@ export class UmbTreeItemChildrenManager<
 		this.targetPagination.clear();
 	}
 
-	async #resetChildren() {
+	/**
+	 * Loads children using offset pagination only.
+	 * This is a "safe" fallback that does NOT:
+	 * - Use target pagination
+	 * - Retry with new targets
+	 * - Call #resetChildren (preventing recursion)
+	 * - Throw errors (fails gracefully)
+	 */
+	async #loadChildrenWithOffsetPagination(): Promise<void> {
+		const repository = this.#treeContext?.getRepository();
+		if (!repository) {
+			// Terminal fallback - fail silently rather than throwing
+			return;
+		}
+
+		this.#isLoading.setValue(true);
+
+		const parent = this.getStartNode() || this.getTreeItem();
+		const foldersOnly = this.getFoldersOnly();
+		const additionalArgs = this.getAdditionalRequestArgs();
+
+		const offsetPaging: UmbOffsetPaginationRequestModel = {
+			skip: 0, // Always from the start
+			take: this.offsetPagination.getPageSize(),
+		};
+
+		const { data } = parent?.unique
+			? await repository.requestTreeItemsOf({
+					parent: { unique: parent.unique, entityType: parent.entityType },
+					skip: offsetPaging.skip,
+					take: offsetPaging.take,
+					paging: offsetPaging,
+					foldersOnly,
+					...additionalArgs,
+				})
+			: await repository.requestTreeRootItems({
+					skip: offsetPaging.skip,
+					take: offsetPaging.take,
+					paging: offsetPaging,
+					foldersOnly,
+					...additionalArgs,
+				});
+
+		if (data) {
+			const items = data.items as Array<TreeItemType>;
+			this.#children.setValue(items);
+			this.setHasChildren(data.total > 0);
+			this.offsetPagination.setTotalItems(data.total);
+		}
+		// Note: On error, we simply don't update state - UI shows stale data
+		// This is the terminal fallback, no further recovery
+
+		this.#isLoading.setValue(false);
+	}
+
+	async #resetChildren(reason: ResetReason = 'error'): Promise<void> {
+		// Clear pagination state
 		this.targetPagination.clear();
 		this.offsetPagination.clear();
-		this.loadChildren();
-		const notificationManager = await this.getContext(UMB_NOTIFICATION_CONTEXT);
-		notificationManager?.peek('danger', { data: { message: 'Menu loading failed. Showing the first items again.' } });
+
+		// Reset retry counters to prevent any lingering retry state
+		this.#loadChildrenRetries = 0;
+		this.#loadPrevItemsRetries = 0;
+		this.#loadNextItemsRetries = 0;
+
+		// Load using offset pagination only - this is our terminal fallback
+		await this.#loadChildrenWithOffsetPagination();
+
+		// Only show notification for actual errors
+		if (reason === 'error') {
+			const notificationManager = await this.getContext(UMB_NOTIFICATION_CONTEXT);
+			notificationManager?.peek('danger', {
+				data: { message: 'Menu loading failed. Showing the first items again.' },
+			});
+		}
 	}
 
 	#onPageChange = () => this.loadNextChildren();

src/Umbraco.Web.UI.Client/src/packages/management-api/tree/tree-data.request-manager.ts

@@ -229,7 +229,10 @@ export class UmbManagementApiTreeDataRequestManager<
 		return args.take !== undefined ? args.take : this.#defaultTakeSize;
 	}
 
-	#getTargetResultHasValidParents(data: Array<TreeItemType>, parentUnique: string | null): boolean {
+	#getTargetResultHasValidParents(data: Array<TreeItemType> | undefined, parentUnique: string | null): boolean {
+		if (!data) {
+			return false;
+		}
 		return data.every((item) => {
 			if (item.parent) {
 				return item.parent.id === parentUnique;