multiple runtime bugfixes for 1.5.5+65

test/widget/route_transition_gate_test.dart

@@ -83,4 +83,19 @@ void main() {
     expect(find.byKey(gated), findsOneWidget);
     expect(find.byKey(placeholder), findsNothing);
   });
+
+  testWidgets('holds the child back for one frame without a route animation', (
+    tester,
+  ) async {
+    // The tab case: no route animation ever runs, but the ancestors (and any
+    // freshly inserted tab-transition Transform) do their first layout in this
+    // very frame, so the fragile subtree must not be part of it.
+    await tester.pumpWidget(MaterialApp(home: gatedPage()));
+    expect(find.byKey(placeholder), findsOneWidget);
+    expect(find.byKey(gated), findsNothing);
+
+    await tester.pump();
+    expect(find.byKey(gated), findsOneWidget);
+    expect(find.byKey(placeholder), findsNothing);
+  });
 }