Fix DropdownMenu keyboard navigation (#147294)

Fix #147253
Fix #147516

Resubmitted #147285 because I accidentally added reviewer as contributor in a commit by using Github "add suggestion to batch". Which causing CLA check to fail and I cannot revert it.

I use exact match of label instead of contains to minimise possible change in actual user search.
I added the new test after the original keyboard navigation test to avoid merge conflict with another incoming PR.
Let me know if I need to make any changes.

Sorry for the inconvenience, I am new to the process.

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

@@ -550,6 +550,9 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
     if (searchText.isEmpty) {
       return null;
     }
+    if (currentHighlight != null && entries[currentHighlight!].label.toLowerCase().contains(searchText)) {
+      return currentHighlight;
+    }
     final int index = entries.indexWhere((DropdownMenuEntry<T> entry) => entry.label.toLowerCase().contains(searchText));
 
     return index != -1 ? index : null;

packages/flutter/test/material/dropdown_menu_test.dart

@@ -764,6 +764,116 @@ void main() {
     expect(item5material.color, Colors.transparent); // the previous item should not be highlighted.
   }, variant: TargetPlatformVariant.desktop());
 
+  // Regression test for https://github.com/flutter/flutter/issues/147253.
+  testWidgets('Down key and up key can navigate on desktop platforms '
+      'when a label text contains another label text', (WidgetTester tester) async {
+    final ThemeData themeData = ThemeData();
+    await tester.pumpWidget(MaterialApp(
+      theme: themeData,
+      home: const Scaffold(
+        body: DropdownMenu<int>(
+          dropdownMenuEntries: <DropdownMenuEntry<int>>[
+            DropdownMenuEntry<int>(
+              value: 0,
+              label: 'ABC'
+            ),
+            DropdownMenuEntry<int>(
+              value: 1,
+              label: 'AB'
+            ),
+            DropdownMenuEntry<int>(
+              value: 2,
+              label: 'ABCD'
+            ),
+          ],
+        ),
+      ),
+    ));
+
+    await tester.tap(find.byType(DropdownMenu<int>));
+    await tester.pump();
+
+    final Finder button0Material = find.descendant(
+      of: find.widgetWithText(MenuItemButton, 'ABC').last,
+      matching: find.byType(Material),
+    );
+    final Finder button1Material = find.descendant(
+      of: find.widgetWithText(MenuItemButton, 'AB').last,
+      matching: find.byType(Material),
+    );
+    final Finder button2Material = find.descendant(
+      of: find.widgetWithText(MenuItemButton, 'ABCD').last,
+      matching: find.byType(Material),
+    );
+
+    // Press down key three times, the highlight should move to the next item each time.
+    await tester.sendKeyEvent(LogicalKeyboardKey.arrowDown);
+    await tester.pumpAndSettle();
+    Material item0Material = tester.widget<Material>(button0Material);
+    expect(item0Material.color, themeData.colorScheme.onSurface.withOpacity(0.12));
+
+    await tester.sendKeyEvent(LogicalKeyboardKey.arrowDown);
+    await tester.pumpAndSettle();
+    Material item1Material = tester.widget<Material>(button1Material);
+    expect(item1Material.color, themeData.colorScheme.onSurface.withOpacity(0.12));
+
+    await tester.sendKeyEvent(LogicalKeyboardKey.arrowDown);
+    await tester.pumpAndSettle();
+    final Material item2Material = tester.widget<Material>(button2Material);
+    expect(item2Material.color, themeData.colorScheme.onSurface.withOpacity(0.12));
+
+    // Press up key two times, the highlight should up each time.
+    await tester.sendKeyEvent(LogicalKeyboardKey.arrowUp);
+    await tester.pumpAndSettle();
+    item1Material = tester.widget<Material>(button1Material);
+    expect(item1Material.color, themeData.colorScheme.onSurface.withOpacity(0.12));
+
+    await tester.sendKeyEvent(LogicalKeyboardKey.arrowUp);
+    await tester.pumpAndSettle();
+    item0Material = tester.widget<Material>(button0Material);
+    expect(item0Material.color, themeData.colorScheme.onSurface.withOpacity(0.12));
+
+  }, variant: TargetPlatformVariant.desktop());
+
+  // Regression test for https://github.com/flutter/flutter/issues/147253.
+  testWidgets('Default search prioritises the current highlight on desktop platforms',
+      (WidgetTester tester) async {
+    final ThemeData themeData = ThemeData();
+    await tester.pumpWidget(MaterialApp(
+      theme: themeData,
+      home: Scaffold(
+        body: DropdownMenu<TestMenu>(
+          dropdownMenuEntries: menuChildren,
+        ),
+      ),
+    ));
+
+    const String itemLabel = 'Item 2';
+    // Open the menu
+    await tester.tap(find.byType(DropdownMenu<TestMenu>));
+    await tester.pumpAndSettle();
+    // Highlight the third item by exact search.
+    await tester.enterText(find.byType(TextField).first, itemLabel);
+    await tester.pumpAndSettle();
+    Finder button2Material = find.descendant(
+      of: find.widgetWithText(MenuItemButton, itemLabel).last,
+      matching: find.byType(Material),
+    );
+    Material item2material = tester.widget<Material>(button2Material);
+    expect(item2material.color, themeData.colorScheme.onSurface.withOpacity(0.12));
+
+    // Search something that matches multiple items.
+    await tester.enterText(find.byType(TextField).first, 'Item');
+    await tester.pumpAndSettle();
+    // The third item should still be highlighted.
+    button2Material = find.descendant(
+      of: find.widgetWithText(MenuItemButton, itemLabel).last,
+      matching: find.byType(Material),
+    );
+    item2material = tester.widget<Material>(button2Material);
+    expect(item2material.color, themeData.colorScheme.onSurface.withOpacity(0.12));
+  }, variant: TargetPlatformVariant.desktop());
+
   testWidgets('The text input should match the label of the menu item '
       'while pressing down key on desktop platforms', (WidgetTester tester) async {
     final ThemeData themeData = ThemeData();