Delay DropdownMenu filtering until text input (#152368)

fixes flutter/flutter#152055

Disabling filtering in `DropdownMenu` at start and after a selection has been made, and re-enable it (if `widget.enableFilter` is true) after text input.
This way it doesn't hide all other options when there is an existing selection.

Note: currently this may crush due to issue flutter/flutter#151878 . Which is not directly related to this PR

Author: PurplePolyhedron

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

@@ -488,7 +488,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
   final GlobalKey _leadingKey = GlobalKey();
   late List<GlobalKey> buttonItemKeys;
   final MenuController _controller = MenuController();
-  late bool _enableFilter;
+  bool _enableFilter = false;
   late List<DropdownMenuEntry<T>> filteredEntries;
   List<Widget>? _initialMenu;
   int? currentHighlight;
@@ -504,7 +504,6 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
     } else {
       _localTextEditingController = TextEditingController();
     }
-    _enableFilter = widget.enableFilter;
     filteredEntries = widget.dropdownMenuEntries;
     buttonItemKeys = List<GlobalKey>.generate(filteredEntries.length, (int index) => GlobalKey());
     _menuHasEnabledItem = filteredEntries.any((DropdownMenuEntry<T> entry) => entry.enabled);
@@ -537,6 +536,11 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
       }
       _localTextEditingController = widget.controller ?? TextEditingController();
     }
+    if (oldWidget.enableFilter != widget.enableFilter) {
+      if (!widget.enableFilter) {
+        _enableFilter = false;
+      }
+    }
     if (oldWidget.enableSearch != widget.enableSearch) {
       if (!widget.enableSearch) {
         currentHighlight = null;
@@ -676,6 +680,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
               );
               currentHighlight = widget.enableSearch ? i : null;
               widget.onSelected?.call(entry.value);
+              _enableFilter = false;
             }
           : null,
         requestFocusOnHover: false,
@@ -748,6 +753,8 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
     if (_enableFilter) {
       filteredEntries = widget.filterCallback?.call(filteredEntries, _localTextEditingController!.text)
         ?? filter(widget.dropdownMenuEntries, _localTextEditingController!);
+    } else {
+      filteredEntries = widget.dropdownMenuEntries;
     }
 
     if (widget.enableSearch) {

packages/flutter/test/material/dropdown_menu_test.dart

@@ -2524,6 +2524,49 @@ void main() {
 
     expect(menuAnchor.alignmentOffset, alignmentOffset);
   });
+
+  testWidgets('DropdownMenu filter is disabled until text input', (WidgetTester tester) async{
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: DropdownMenu<TestMenu>(
+          requestFocusOnTap: true,
+          enableFilter: true,
+          initialSelection: menuChildren[0].value,
+          dropdownMenuEntries: menuChildren,
+        ),
+      ),
+    ));
+
+    await tester.tap(find.byType(DropdownMenu<TestMenu>));
+    await tester.pumpAndSettle();
+
+    // All entries should be available, and two buttons should be found for each entry.
+    // One is layout for the _DropdownMenuBody, the other one is the real button item in the menu.
+    for (final TestMenu menu in TestMenu.values) {
+      expect(find.widgetWithText(MenuItemButton, menu.label), findsNWidgets(2));
+    }
+
+    // Text input would enable the filter.
+    await tester.enterText(find.byType(TextField).first, 'Menu 1');
+    await tester.pumpAndSettle();
+    for (final TestMenu menu in TestMenu.values) {
+      // 'Menu 1' should be 2, other items should only find one.
+      if (menu.label == TestMenu.mainMenu1.label) {
+        expect(find.widgetWithText(MenuItemButton, menu.label), findsNWidgets(2));
+      } else {
+        expect(find.widgetWithText(MenuItemButton, menu.label), findsOneWidget);
+      }
+    }
+
+    // Selecting an item would disable filter again.
+    await tester.tap(find.widgetWithText(MenuItemButton, 'Menu 1').last);
+    await tester.pumpAndSettle();
+    await tester.tap(find.byType(DropdownMenu<TestMenu>));
+    await tester.pumpAndSettle();
+    for (final TestMenu menu in TestMenu.values) {
+      expect(find.widgetWithText(MenuItemButton, menu.label), findsNWidgets(2));
+    }
+  });
 }
 
 enum TestMenu {