🐛 : FIX : Long labels overflowing in extended navigation rail. (#145474)

It has been observed that while in extended navigation rail, if longer labels are given the it is overflowing. Problem identified in "NavigationRail" widget only in case of extended version of it. This PR includes the fix for the same.

Fixes flutter/flutter#110901.

*NO breaking changes.*

packages/flutter/lib/src/material/navigation_rail.dart

@@ -698,16 +698,19 @@ class _RailDestinationState extends State<_RailDestination> {
               ),
               child: ClipRect(
                 child: Row(
+                  mainAxisSize: MainAxisSize.min,
                   children: <Widget>[
                     iconPart,
-                    Align(
-                      heightFactor: 1.0,
-                      widthFactor: widget.extendedTransitionAnimation.value,
-                      alignment: AlignmentDirectional.centerStart,
-                      child: FadeTransition(
-                        alwaysIncludeSemantics: true,
-                        opacity: labelFadeAnimation,
-                        child: styledLabel,
+                    Flexible(
+                      child: Align(
+                        heightFactor: 1.0,
+                        widthFactor: widget.extendedTransitionAnimation.value,
+                        alignment: AlignmentDirectional.centerStart,
+                        child: FadeTransition(
+                          alwaysIncludeSemantics: true,
+                          opacity: labelFadeAnimation,
+                          child: styledLabel,
+                        ),
                       ),
                     ),
                     SizedBox(width: _horizontalDestinationPadding * widget.extendedTransitionAnimation.value),

packages/flutter/test/material/navigation_rail_test.dart

@@ -7,6 +7,7 @@ import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
+
 import '../widgets/semantics_tester.dart';
 
 void main() {
@@ -3631,6 +3632,55 @@ void main() {
     expect(tester.takeException(), isNull);
   });
 
+  testWidgets('NavigationRail labels shall not overflow if longer texts provided - extended', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/110901.
+    // The navigation rail has a narrow width constraint. The text should wrap.
+    const String normalLabel = 'Abc';
+    const String longLabel = 'Very long bookmark text for navigation destination';
+    await tester.pumpWidget(MaterialApp(
+      home: Builder(
+        builder: (BuildContext context) {
+          return Scaffold(
+            body: Row(
+              children: <Widget>[
+                SizedBox(
+                  width: 140.0,
+                  child: NavigationRail(
+                    selectedIndex: 1,
+                    extended: true,
+                    destinations: const <NavigationRailDestination>[
+                      NavigationRailDestination(
+                        icon: Icon(Icons.favorite_border),
+                        selectedIcon: Icon(Icons.favorite),
+                        label: Text(normalLabel),
+                      ),
+                      NavigationRailDestination(
+                        icon: Icon(Icons.bookmark_border),
+                        selectedIcon: Icon(Icons.bookmark),
+                        label: Text(longLabel),
+                      ),
+                    ],
+                  ),
+                ),
+                const Expanded(
+                  child: Text('body'),
+                ),
+              ],
+            ),
+          );
+        },
+      ),
+    ));
+
+    expect(find.byType(NavigationRail), findsOneWidget);
+    expect(find.text(normalLabel), findsOneWidget);
+    expect(find.text(longLabel), findsOneWidget);
+
+    // If the widget manages to layout without throwing an overflow exception,
+    // the test passes.
+    expect(tester.takeException(), isNull);
+  });
+
   group('Material 2', () {
     // These tests are only relevant for Material 2. Once Material 2
     // support is deprecated and the APIs are removed, these tests