Fix overflow issue caused by a long title/subtitle for the vertical stepper (#61623)

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

@@ -520,9 +520,11 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
               _buildLine(!_isLast(index)),
             ],
           ),
-          Container(
-            margin: const EdgeInsetsDirectional.only(start: 12.0),
-            child: _buildHeaderText(index),
+          Expanded(
+            child: Container(
+              margin: const EdgeInsetsDirectional.only(start: 12.0),
+              child: _buildHeaderText(index),
+            )
           ),
         ],
       ),

packages/flutter/test/material/stepper_test.dart

@@ -663,4 +663,57 @@ void main() {
     await tester.pump();
     expect(disabledNode.hasPrimaryFocus, isFalse);
   });
+
+  testWidgets('Stepper header title should not overflow', (WidgetTester tester) async {
+    const String longText =
+        'A long long long long long long long long long long long long text';
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: ListView(
+            children: <Widget>[
+              Stepper(
+                steps: const <Step>[
+                  Step(
+                    title: Text(longText),
+                    content: Text('Text content')
+                  ),
+                ],
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isNull);
+  });
+
+  testWidgets('Stepper header subtitle should not overflow', (WidgetTester tester) async {
+    const String longText =
+        'A long long long long long long long long long long long long text';
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: ListView(
+            children: <Widget>[
+              Stepper(
+                steps: const <Step>[
+                  Step(
+                    title: Text('Regular title'),
+                    subtitle: Text(longText),
+                    content: Text('Text content')
+                  ),
+                ],
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isNull);
+  });
 }