Allow paneBodyBuilder to use an IndexedStack without interference

CHANGELOG.md

@@ -1,5 +1,7 @@
-## next
+## (future)
 
+- Fix assertion in `NavigationViewState` if no pane was currently selected ([#678](https://github.com/bdlukaa/fluent_ui/issues/678))
+- Make `NavigationView.paneBodyBuilder` responsible for state management of the widget it returns, allowing `paneBodyBuilder` to return an `IndexedStack` (common use case) ([#679](https://github.com/bdlukaa/fluent_ui/issues/679))
 - Add `AutoSuggestBox.maxPopupHeight` ([#677](https://github.com/bdlukaa/fluent_ui/issues/677))
 
 ## 4.1.4

lib/src/controls/navigation/navigation_view/body.dart

@@ -140,27 +140,39 @@ class _NavigationBodyState extends State<_NavigationBody> {
             child: child,
           );
         },
-        child: KeyedSubtree(
-          key: widget.itemKey,
-          child: PageView.builder(
-            key: _pageKey,
-            physics: const NeverScrollableScrollPhysics(),
-            controller: pageController,
-            itemCount: view.pane!.effectiveItems.length,
-            itemBuilder: (context, index) {
-              final isSelected = view.pane!.selected == index;
-              final item = view.pane!.effectiveItems[index];
-
-              return ExcludeFocus(
-                key: item.bodyKey,
-                excluding: !isSelected,
-                child: FocusTraversalGroup(
-                  child: widget.paneBodyBuilder?.call(item.body) ?? item.body,
-                ),
-              );
-            },
-          ),
-        ),
+        child: () {
+          final paneBodyBuilder = widget.paneBodyBuilder;
+          if (paneBodyBuilder != null) {
+            return FocusTraversalGroup(
+              key: widget.itemKey,
+              child: paneBodyBuilder.call(view.pane?.selected != null
+                  ? view.pane?.selectedItem.body
+                  : null),
+            );
+          } else {
+            return KeyedSubtree(
+              key: widget.itemKey,
+              child: PageView.builder(
+                key: _pageKey,
+                physics: const NeverScrollableScrollPhysics(),
+                controller: pageController,
+                itemCount: view.pane!.effectiveItems.length,
+                itemBuilder: (context, index) {
+                  final isSelected = view.pane!.selected == index;
+                  final item = view.pane!.effectiveItems[index];
+
+                  return ExcludeFocus(
+                    key: item.bodyKey,
+                    excluding: !isSelected,
+                    child: FocusTraversalGroup(
+                      child: item.body,
+                    ),
+                  );
+                },
+              ),
+            );
+          }
+        }(),
       ),
     );
   }

lib/src/controls/navigation/navigation_view/view.dart

@@ -64,6 +64,10 @@ class NavigationView extends StatefulWidget {
   /// and the body of the navigation pane is dynamically determined or
   /// affected by the current route rather than just by the currently
   /// selected pane.
+  ///
+  /// If this is not null then this builder will be responsible for state
+  /// management of the child widget. One way to accomplish this is to
+  /// use an [IndexedStack].
   final NavigationContentBuilder? paneBodyBuilder;
 
   /// The navigation pane, that can be displayed either on the
@@ -214,7 +218,9 @@ class NavigationViewState extends State<NavigationView> {
     if (oldWidget.pane?.selected != widget.pane?.selected) {
       _oldIndex = oldWidget.pane?.selected ?? -1;
 
-      final item = widget.pane?.selectedItem.itemKey.currentContext;
+      final item = widget.pane?.selected == null
+          ? null
+          : widget.pane?.selectedItem.itemKey.currentContext;
 
       if (item != null) {
         final atEnd =