enhanced route transition gate because of a syncfusion viewer bug

lib/widget/file_viewer/deferred_pdf_viewer.dart

@@ -5,8 +5,9 @@ import 'package:syncfusion_flutter_pdfviewer/pdfviewer.dart';
 
 import '../route_transition_gate.dart';
 
-/// SfPdfViewer asserts on `localToGlobal` if mounted during the page-push
-/// animation. Defer until the route enter animation completes.
+/// SfPdfViewer asserts on `localToGlobal` if laid out while a route
+/// transition's fresh `RenderTransform` has no size yet. Mount it only while
+/// the route is at rest (see [RouteTransitionGate]).
 class DeferredPdfViewer extends StatelessWidget {
   const DeferredPdfViewer({super.key, required this.path});
   final String path;