Retention of scroll offset in emoji

lib/widgets/autocomplete.dart

@@ -38,6 +38,7 @@ abstract class AutocompleteField<QueryT extends AutocompleteQuery, ResultT exten
 
 class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends AutocompleteResult> extends State<AutocompleteField<QueryT, ResultT>> with PerAccountStoreAwareStateMixin<AutocompleteField<QueryT, ResultT>> {
   AutocompleteView<QueryT, ResultT>? _viewModel;
+  final ScrollController _scrollController = ScrollController();
 
   void _initViewModel(QueryT query) {
     _viewModel = widget.initViewModel(context, query)
@@ -63,6 +64,10 @@ class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends
       } else {
         assert(_viewModel!.acceptsQuery(newQuery));
         _viewModel!.query = newQuery;
+        // Reset scroll when query content changes
+        if (_scrollController.hasClients) {
+          _scrollController.jumpTo(0);
+        }
       }
     }
   }
@@ -96,6 +101,7 @@ class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends
   void dispose() {
     widget.controller.removeListener(_handleControllerChange);
     _viewModel?.dispose(); // removes our listener
+    _scrollController.dispose();
     super.dispose();
   }
 
@@ -138,6 +144,7 @@ class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends
             child: ConstrainedBox(
               constraints: const BoxConstraints(maxHeight: 300), // TODO not hard-coded
               child: ListView.builder(
+                controller: _scrollController,
                 padding: EdgeInsets.zero,
                 shrinkWrap: true,
                 itemCount: _resultsToDisplay.length,

test/widgets/autocomplete_test.dart

@@ -503,6 +503,72 @@ void main() {
 
       debugNetworkImageHttpClientProvider = null;
     });
+
+    testWidgets('scroll position resets when query changes', (tester) async {
+      final composeInputFinder = await setupToComposeInput(tester);
+      final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+      // Add many emoji starting with 's' to ensure the list is scrollable.
+      // The autocomplete box has maxHeight of 300, so we need enough items.
+      store.setServerEmojiData(ServerEmojiData(codeToNames: {
+        '1f600': ['sa_first_item'],
+        '1f601': ['sb_second'],
+        '1f602': ['sc_third'],
+        '1f603': ['sd_fourth'],
+        '1f604': ['se_fifth'],
+        '1f605': ['sf_sixth'],
+        '1f606': ['sg_seventh'],
+        '1f607': ['sh_eighth'],
+        '1f608': ['si_ninth'],
+        '1f609': ['sj_tenth'],
+        '1f60a': ['sk_eleventh'],
+        '1f60b': ['sl_twelfth'],
+        '1f60c': ['sm_thirteenth'],
+        '1f60d': ['sn_fourteenth'],
+        '1f60e': ['so_fifteenth'],
+        '1f60f': ['sp_sixteenth'],
+        '1f610': ['sq_seventeenth'],
+        '1f611': ['sr_eighteenth'],
+        '1f612': ['ss_nineteenth'],
+        '1f613': ['sz_last_item'],
+      }));
+
+      // Enter an emoji query to show many options.
+      // TODO(#226): Remove this extra edit when this bug is fixed.
+      await tester.enterText(composeInputFinder, 'hi :');
+      await tester.enterText(composeInputFinder, 'hi :s');
+      await tester.pump();
+      await tester.pump();
+
+      // Verify the first item is visible and the last is not (needs scrolling).
+      check(find.text('sa_first_item')).findsOne();
+
+      // Find the autocomplete ListView's Scrollable.
+      final listViewFinder = find.descendant(
+        of: find.byType(Material),
+        matching: find.byType(ListView),
+      );
+      final scrollableFinder = find.descendant(
+        of: listViewFinder,
+        matching: find.byType(Scrollable));
+
+      // Scroll until the last item is visible.
+      await tester.scrollUntilVisible(find.text('sz_last_item'), 50,
+        scrollable: scrollableFinder);
+      await tester.pump();
+
+      // The last item should now be visible.
+      check(find.text('sz_last_item')).findsOne();
+
+      // Change the query; scroll should reset to top.
+      await tester.enterText(composeInputFinder, 'hi :sa');
+      await tester.pump();
+      await tester.pump();
+
+      // After query change, the first matching item should be visible at top.
+      check(find.text('sa_first_item')).findsOne();
+
+      debugNetworkImageHttpClientProvider = null;
+    });
   });
 
   group('TopicAutocomplete', () {