♻️ refactor(example): wrap threshold marker in Positioned.fill

Ensure the grid mode threshold marker properly fills its parent
container by wrapping LayoutBuilder in Positioned.fill.

This improves layout consistency and prevents potential
positioning issues during scale operations.

Author: iota9star

example/lib/pages/scalable_flexbox_page.dart

@@ -400,23 +400,31 @@ class _ScalableFlexboxPageState extends State<ScalableFlexboxPage>
                         ),
                       ),
                       // Mode switch threshold marker
-                      LayoutBuilder(
-                        builder: (context, constraints) {
-                          final thresholdFraction =
-                              (_gridModeThreshold -
-                                  _scaleController.minExtent) /
-                              (_scaleController.maxExtent -
-                                  _scaleController.minExtent);
-                          return Positioned(
-                            left: constraints.maxWidth * thresholdFraction - 1,
-                            top: 0,
-                            bottom: 0,
-                            child: Container(
-                              width: 2,
-                              color: NeoBrutalism.orange,
-                            ),
-                          );
-                        },
+                      Positioned.fill(
+                        child: LayoutBuilder(
+                          builder: (context, constraints) {
+                            final thresholdFraction =
+                                (_gridModeThreshold -
+                                    _scaleController.minExtent) /
+                                (_scaleController.maxExtent -
+                                    _scaleController.minExtent);
+                            return Stack(
+                              children: [
+                                Positioned(
+                                  left:
+                                      constraints.maxWidth * thresholdFraction -
+                                      1,
+                                  top: 0,
+                                  bottom: 0,
+                                  child: Container(
+                                    width: 2,
+                                    color: NeoBrutalism.orange,
+                                  ),
+                                ),
+                              ],
+                            );
+                          },
+                        ),
                       ),
                     ],
                   ),