Remove FocusTraversalGroups from the final sorted list of descendants. (#74758)

This fixes a subtle bug in the focus traversal that would leave unfocusable FocusTraversalGroup nodes in the list of sorted nodes to be traversed, causing an assert to fire if you nested a FocusTraversalGroup in another one and made its children unfocusable.

packages/flutter/lib/src/widgets/focus_traversal.dart

@@ -335,10 +335,23 @@ abstract class FocusTraversalPolicy with Diagnosticable {
       }
     }
 
+    // Visit the children of the scope.
     visitGroups(groups[scopeGroupMarker?.focusNode]!);
+
+    // Remove the FocusTraversalGroup nodes themselves, which aren't focusable.
+    // They were left in above because they were needed to find their members
+    // during sorting.
+    sortedDescendants.removeWhere((FocusNode node) {
+      return !node.canRequestFocus || node.skipTraversal;
+    });
+
+    // Sanity check to make sure that the algorithm above doesn't diverge from
+    // the one in FocusScopeNode.traversalDescendants in terms of which nodes it
+    // finds.
     assert(
       sortedDescendants.length <= scope.traversalDescendants.length && sortedDescendants.toSet().difference(scope.traversalDescendants.toSet()).isEmpty,
-      'sorted descendants contains more nodes than it should: (${sortedDescendants.toSet().difference(scope.traversalDescendants.toSet())})'
+      'Sorted descendants contains different nodes than FocusScopeNode.traversalDescendants would. '
+      'These are the different nodes: ${sortedDescendants.toSet().difference(scope.traversalDescendants.toSet())}'
     );
     return sortedDescendants;
   }

packages/flutter/test/widgets/focus_traversal_test.dart

@@ -2088,6 +2088,82 @@ void main() {
       expect(containerNode.hasFocus, isFalse);
       expect(unfocusableNode.hasFocus, isFalse);
     });
+    testWidgets("Nested FocusTraversalGroup with unfocusable children doesn't assert.", (WidgetTester tester) async {
+      final GlobalKey key1 = GlobalKey(debugLabel: '1');
+      final GlobalKey key2 = GlobalKey(debugLabel: '2');
+      final FocusNode focusNode = FocusNode();
+      bool? gotFocus;
+      await tester.pumpWidget(
+        FocusTraversalGroup(
+          child: Column(
+            children: <Widget>[
+              Focus(
+                autofocus: true,
+                child: Container(),
+              ),
+              FocusTraversalGroup(
+                descendantsAreFocusable: false,
+                child: Focus(
+                  onFocusChange: (bool focused) => gotFocus = focused,
+                  child: Focus(
+                    key: key1,
+                    focusNode: focusNode,
+                    child: Container(key: key2),
+                  ),
+                ),
+              ),
+            ],
+          ),
+        ),
+      );
+
+      final Element childWidget = tester.element(find.byKey(key1));
+      final FocusNode unfocusableNode = Focus.of(childWidget);
+      final Element containerWidget = tester.element(find.byKey(key2));
+      final FocusNode containerNode = Focus.of(containerWidget);
+
+      await tester.pump();
+      primaryFocus!.nextFocus();
+
+      expect(gotFocus, isNull);
+      expect(containerNode.hasFocus, isFalse);
+      expect(unfocusableNode.hasFocus, isFalse);
+
+      containerNode.requestFocus();
+      await tester.pump();
+
+      expect(gotFocus, isNull);
+      expect(containerNode.hasFocus, isFalse);
+      expect(unfocusableNode.hasFocus, isFalse);
+    });
+    testWidgets("Empty FocusTraversalGroup doesn't cause an exception.", (WidgetTester tester) async {
+      final GlobalKey key = GlobalKey(debugLabel: 'Test Key');
+      final FocusNode focusNode = FocusNode(debugLabel: 'Test Node');
+      await tester.pumpWidget(
+        FocusTraversalGroup(
+          child: Directionality(
+            textDirection: TextDirection.rtl,
+            child: Column(
+              children: <Widget>[
+                FocusTraversalGroup(
+                  child: Container(key: key),
+                ),
+                Focus(
+                  focusNode: focusNode,
+                  autofocus: true,
+                  child: Container(),
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+
+      await tester.pump();
+      primaryFocus!.nextFocus();
+      await tester.pump();
+      expect(primaryFocus, equals(focusNode));
+    });
   });
   group(RawKeyboardListener, () {
     testWidgets('Raw keyboard listener introduces a Semantics node by default', (WidgetTester tester) async {