Backup: fix file-browser tree-state, indeterminate, and item count

Three related bugs in the modernized file-browser selection:

- Deselecting a row inside a selected subtree only added an exception
  but never propagated up. Now walks up: when every loaded child of an
  ancestor is effectively deselected, the ancestor is deselected too
  (and re-selecting reverses the same walk: when every loaded child
  becomes effectively selected, the ancestor follows). Stops at the
  first ancestor with un-loaded children.
- Indeterminate only fired for selected folders with a deselected
  descendant. Now symmetric: an unselected folder with a selected
  descendant also reads as indeterminate (e.g. visitor checks a single
  leaf without its ancestors).
- `applySelect` cleared an own-negative entry but didn't add an own
  positive, so re-checking a path whose ancestor stayed in `deselected`
  would silently inherit unchecked. Now: drop the negative and, only
  if removing it wouldn't already leave the row inherited-selected
  from an ancestor, add the explicit positive.
- The header "selected" count tracked `selected.size` (the explicit
  positives), which never grew as the visitor expanded an inherited
  subtree. Now hoists loaded folder children to the FileBrowser via a
  per-row callback and counts effectively-selected leaves only: a file
  counts as 1, a folder counts as 1 only when its contents haven't
  been loaded yet (otherwise the server treats it as one opaque
  download unit), and indeterminate folders contribute only the
  effectively-selected descendants underneath. BackupDetail reads the
  same count for its Download / Restore action labels via a new
  `onSelectionCountChange` callback, and the labels switch to proper
  singular/plural via `_n()`.

`useMockFileTree` no longer seeds non-null callers with the root tree
on first render — the seeding leaked the root tree into closed
NodeRows' `children` state and let the registerChildren effect
accidentally register MOCK_FILE_TREE for non-root paths during the
window between "open clicked" and "fetch resolved", which infinitely
recursed the count walk. FileBrowser now imports `MOCK_FILE_TREE`
directly for its roots; the hook handles only lazy non-root loads.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dhasilva

projects/packages/backup/src/dashboard/components/backup-detail/index.tsx

@@ -1,52 +1,52 @@
 import { dateI18n } from '@wordpress/date';
 import { useState } from '@wordpress/element';
-import { __, sprintf } from '@wordpress/i18n';
+import { __, _n, sprintf } from '@wordpress/i18n';
 import { Icon, cloud, download as downloadIcon, rotateLeft } from '@wordpress/icons';
 import { Link } from '@wordpress/route';
 import { Card, Stack, Text } from '@wordpress/ui';
 import FileBrowser, { EMPTY_FILE_SELECTION } from '../file-browser';
 import './style.scss';
-import type { FileSelection } from '../file-browser';
 import type { BackupActivityItem } from '../../types/activity';
+import type { FileSelection } from '../file-browser';
 
 type Props = {
 	item: BackupActivityItem;
 };
 
 /**
- * Returns the appropriate "Download" header-label given how many files
+ * Returns the appropriate "Download" header-label given how many items
  * the visitor has selected in the file browser. With zero selections we
  * default to the whole-backup download; otherwise we count the selected
- * paths.
+ * items (files + folders) currently visible in the loaded tree.
  *
- * @param count - Number of currently selected files/folders.
+ * @param count - Number of currently selected items.
  * @return Localized button label.
  */
 function downloadLabel( count: number ): string {
 	if ( count === 0 ) {
 		return __( 'Download backup', 'jetpack-backup-pkg' );
 	}
 	return sprintf(
-		/* translators: %d count of selected files */
-		__( 'Download %d selected files', 'jetpack-backup-pkg' ),
+		/* translators: %d count of selected items (files + opaque folders) */
+		_n( 'Download %d selected item', 'Download %d selected items', count, 'jetpack-backup-pkg' ),
 		count
 	);
 }
 
 /**
- * Returns the appropriate "Restore" header-label given how many files
+ * Returns the appropriate "Restore" header-label given how many items
  * the visitor has selected in the file browser.
  *
- * @param count - Number of currently selected files/folders.
+ * @param count - Number of currently selected items.
  * @return Localized button label.
  */
 function restoreLabel( count: number ): string {
 	if ( count === 0 ) {
 		return __( 'Restore to this point', 'jetpack-backup-pkg' );
 	}
 	return sprintf(
-		/* translators: %d count of selected files */
-		__( 'Restore %d selected files', 'jetpack-backup-pkg' ),
+		/* translators: %d count of selected items (files + opaque folders) */
+		_n( 'Restore %d selected item', 'Restore %d selected items', count, 'jetpack-backup-pkg' ),
 		count
 	);
 }
@@ -66,11 +66,12 @@ function restoreLabel( count: number ): string {
  */
 export default function BackupDetail( { item }: Props ) {
 	const [ selection, setSelection ] = useState< FileSelection >( EMPTY_FILE_SELECTION );
-	// "X selected files" in the header reflects the positive set only —
-	// negative exceptions on a folder's subtree don't bump the count
-	// up, so the label always matches what the visitor explicitly asked
-	// for ("you picked N things, here are the exceptions inside them").
-	const count = selection.selected.size;
+	// `count` tracks the number of items the visitor sees ticked in the
+	// loaded subtree (selections + inherited descendants + indeterminate
+	// folders). FileBrowser owns the loaded children, so it reports the
+	// count back here for the header labels to swap between "Download
+	// backup" and "Download N items".
+	const [ count, setCount ] = useState( 0 );
 
 	return (
 		<Card.Root className="jpb-backup-detail">
@@ -112,6 +113,7 @@ export default function BackupDetail( { item }: Props ) {
 						rewindId={ item.rewindId }
 						selection={ selection }
 						onSelectionChange={ setSelection }
+						onSelectionCountChange={ setCount }
 					/>
 				</div>
 			</Card.Content>