[material_ui] Fix DropdownMenuFormField missing leading icon

packages/material_ui/lib/src/dropdown_menu.dart

@@ -421,6 +421,9 @@ class DropdownMenu<T> extends StatefulWidget {
 
   /// The builder function used to create the [InputDecoration] passed to the text field.
   ///
+  /// If the resulting [InputDecoration.prefixIcon] is null, [leadingIcon] is
+  /// assigned as the prefix icon.
+  ///
   /// If a value is provided for this property and the resulting [InputDecoration.suffixIcon]
   /// is null, a default [IconButton] is assigned as the suffix icon. This button's icon will
   /// use [trailingIcon] and [selectedTrailingIcon] if those are explicitly defined; otherwise,
@@ -1266,6 +1269,9 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
         final DropdownMenuDecorationBuilder decorationBuilder =
             widget.decorationBuilder ?? _buildDefaultDecoration;
         InputDecoration decoration = decorationBuilder(context, controller);
+        if (widget.leadingIcon != null && decoration.prefixIcon == null) {
+          decoration = decoration.copyWith(prefixIcon: widget.leadingIcon);
+        }
         // If no suffixIcon is provided, the default IconButton is used for convenience.
         if (decoration.suffixIcon == null) {
           decoration = decoration.copyWith(

packages/material_ui/pending_changelogs/change_2026_09_04_dropdown_menu_leading_icon.yaml

@@ -0,0 +1,3 @@
+changelog: |
+  - Fixes `DropdownMenuFormField.leadingIcon` not being shown in the text field.
+version: patch

packages/material_ui/test/dropdown_menu_form_field_test.dart

@@ -175,6 +175,32 @@ void main() {
     expect(dropdownMenu.leadingIcon, leadingIcon);
   });
 
+  testWidgets('leadingIcon is shown in the text field prefix decoration', (
+    WidgetTester tester,
+  ) async {
+    const Widget leadingIcon = Icon(Icons.check);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: DropdownMenuFormField<MenuItem>(
+            leadingIcon: leadingIcon,
+            dropdownMenuEntries: menuEntries,
+          ),
+        ),
+      ),
+    );
+
+    final TextField textField = tester.widget(find.byType(TextField));
+    final Widget? prefixIcon = textField.decoration?.prefixIcon;
+    expect(prefixIcon, isNotNull);
+    final Finder prefixCheckIcon = find.descendant(
+      of: find.byWidget(prefixIcon!),
+      matching: find.byIcon(Icons.check),
+    );
+    expect(prefixCheckIcon, findsOneWidget);
+  });
+
   testWidgets('Passes trailingIcon to underlying DropdownMenu', (WidgetTester tester) async {
     await tester.pumpWidget(
       MaterialApp(