Fix DropdownMenu default width does not take label into account (#161219)

## Description

This PR fixes `DropdownMenu` default width when a long label is
provided.

Before:

The width is based on the longest menu item (which can be smaller than
the label):


![image](https://github.com/user-attachments/assets/c8d543a3-12c6-4f8e-ba89-e067a09f8653)

After:

The width also depends on the label width when it is longer than the
menu items.


![image](https://github.com/user-attachments/assets/afde7e35-1da0-46a9-900d-8fd119c30317)


## Related Issue

Fixes [DropdownMenu default width does not take label into
account](https://github.com/flutter/flutter/issues/136678)

## Tests

Adds 2 tests.

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

@@ -1079,11 +1079,24 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
                 ? textField
                 : _DropdownMenuBody(
                   width: widget.width,
+                  // The children, except the text field, are used to compute the preferred width,
+                  // which is the width of the longest children, plus the width of trailingButton
+                  // and leadingButton.
+                  //
+                  // See _RenderDropdownMenuBody layout logic.
                   children: <Widget>[
                     textField,
                     ..._initialMenu!.map(
                       (Widget item) => ExcludeFocus(excluding: !controller.isOpen, child: item),
                     ),
+                    if (widget.label != null)
+                      ExcludeSemantics(
+                        child: Padding(
+                          // See RenderEditable.floatingCursorAddedMargin for the default horizontal padding.
+                          padding: const EdgeInsets.symmetric(horizontal: 4.0),
+                          child: DefaultTextStyle(style: effectiveTextStyle!, child: widget.label!),
+                        ),
+                      ),
                     trailingButton,
                     leadingButton,
                   ],
@@ -1317,9 +1330,11 @@ class _RenderDropdownMenuBody extends RenderBox
         continue;
       }
       final double maxIntrinsicWidth = child.getMinIntrinsicWidth(height);
+      // Add the width of leading icon.
       if (child == lastChild) {
         width += maxIntrinsicWidth;
       }
+      // Add the width of trailing icon.
       if (child == childBefore(lastChild!)) {
         width += maxIntrinsicWidth;
       }
@@ -1344,11 +1359,11 @@ class _RenderDropdownMenuBody extends RenderBox
         continue;
       }
       final double maxIntrinsicWidth = child.getMaxIntrinsicWidth(height);
-      // Add the width of leading Icon.
+      // Add the width of leading icon.
       if (child == lastChild) {
         width += maxIntrinsicWidth;
       }
-      // Add the width of trailing Icon.
+      // Add the width of trailing icon.
       if (child == childBefore(lastChild!)) {
         width += maxIntrinsicWidth;
       }

packages/flutter/test/material/dropdown_menu_test.dart

@@ -718,6 +718,64 @@ void main() {
     expect(box.size.width, customWidth);
   });
 
+  testWidgets('The width is determined by the menu entries', (WidgetTester tester) async {
+    const double entryLabelWidth = 100;
+
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          body: DropdownMenu<int>(
+            dropdownMenuEntries: <DropdownMenuEntry<int>>[
+              DropdownMenuEntry<int>(
+                value: 0,
+                label: 'Flutter',
+                labelWidget: SizedBox(width: entryLabelWidth),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final double width = tester.getSize(find.byType(DropdownMenu<int>)).width;
+    const double menuEntryPadding = 24.0; // See _kDefaultHorizontalPadding.
+    const double leadingWidth = 16.0;
+    const double trailingWidth = 56.0;
+
+    expect(width, entryLabelWidth + leadingWidth + trailingWidth + menuEntryPadding);
+  });
+
+  testWidgets('The width is determined by the label when it is longer than menu entries', (
+    WidgetTester tester,
+  ) async {
+    const double labelWidth = 120;
+    const double entryLabelWidth = 100;
+
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          body: DropdownMenu<int>(
+            label: SizedBox(width: labelWidth),
+            dropdownMenuEntries: <DropdownMenuEntry<int>>[
+              DropdownMenuEntry<int>(
+                value: 0,
+                label: 'Flutter',
+                labelWidget: SizedBox(width: entryLabelWidth),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final double width = tester.getSize(find.byType(DropdownMenu<int>)).width;
+    const double leadingWidth = 16.0;
+    const double trailingWidth = 56.0;
+    const double labelPadding = 8.0; // See RenderEditable.floatingCursorAddedMargin.
+
+    expect(width, labelWidth + labelPadding + leadingWidth + trailingWidth);
+  });
+
   testWidgets('The width of MenuAnchor respects MenuAnchor.expandedInsets', (
     WidgetTester tester,
   ) async {
@@ -962,7 +1020,7 @@ void main() {
     // Default text field (without leading icon).
     await tester.pumpWidget(buildTest(themeData, menuChildren, label: const Text('label')));
 
-    final Finder label = find.text('label');
+    final Finder label = find.text('label').first;
     final Offset labelTopLeft = tester.getTopLeft(label);
 
     await tester.tap(find.byType(DropdownMenu<TestMenu>));
@@ -985,7 +1043,7 @@ void main() {
 
     final Finder leadingIcon = find.widgetWithIcon(SizedBox, Icons.search).last;
     final double iconWidth = tester.getSize(leadingIcon).width;
-    final Finder updatedLabel = find.text('label');
+    final Finder updatedLabel = find.text('label').first;
     final Offset updatedLabelTopLeft = tester.getTopLeft(updatedLabel);
 
     await tester.tap(find.byType(DropdownMenu<TestMenu>));
@@ -1009,7 +1067,7 @@ void main() {
 
     final Finder largeLeadingIcon = find.widgetWithIcon(SizedBox, Icons.search).last;
     final double largeIconWidth = tester.getSize(largeLeadingIcon).width;
-    final Finder updatedLabel1 = find.text('label');
+    final Finder updatedLabel1 = find.text('label').first;
     final Offset updatedLabelTopLeft1 = tester.getTopLeft(updatedLabel1);
 
     await tester.tap(find.byType(DropdownMenu<TestMenu>));
@@ -1040,7 +1098,7 @@ void main() {
       ),
     );
 
-    final Finder label = find.text('label');
+    final Finder label = find.text('label').first;
     final Offset labelTopRight = tester.getTopRight(label);
 
     await tester.tap(find.byType(DropdownMenu<TestMenu>));
@@ -1072,7 +1130,7 @@ void main() {
     final Finder leadingIcon = find.widgetWithIcon(SizedBox, Icons.search).last;
     final double iconWidth = tester.getSize(leadingIcon).width;
     final Offset dropdownMenuTopRight = tester.getTopRight(find.byType(DropdownMenu<TestMenu>));
-    final Finder updatedLabel = find.text('label');
+    final Finder updatedLabel = find.text('label').first;
     final Offset updatedLabelTopRight = tester.getTopRight(updatedLabel);
 
     await tester.tap(find.byType(DropdownMenu<TestMenu>));
@@ -1110,7 +1168,7 @@ void main() {
     final Offset updatedDropdownMenuTopRight = tester.getTopRight(
       find.byType(DropdownMenu<TestMenu>),
     );
-    final Finder updatedLabel1 = find.text('label');
+    final Finder updatedLabel1 = find.text('label').first;
     final Offset updatedLabelTopRight1 = tester.getTopRight(updatedLabel1);
 
     await tester.tap(find.byType(DropdownMenu<TestMenu>));