autocomplete: Put best matches near input field.

This commit reverses the list that was originally
presented to the user while showing the typeahead menu.

This makes sense since on mobile its easier to click
on options closer to the input box, i.e.
where your fingers are currently present,
instead of pressing arrow keys on a keyboard which is
true on a desktop setup.

Hence we place the best matching options
not at the top of the typeahead menu, but
instead put them at the bottom for better
reachability and convenience of the user.

Tests have been added to verify the
emoji and mention render behavior.

Also mentions dependencies on zulip#226 where
required.

Co-authored-by: Zixuan James Li zixuan@zulip.com

Fixes zulip#1121.

lib/widgets/autocomplete.dart

@@ -134,6 +134,7 @@ class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends
               constraints: const BoxConstraints(maxHeight: 300), // TODO not hard-coded
               child: ListView.builder(
                 padding: EdgeInsets.zero,
+                reverse: true,
                 shrinkWrap: true,
                 itemCount: _resultsToDisplay.length,
                 itemBuilder: _buildItem))));

test/widgets/autocomplete_test.dart

@@ -177,6 +177,36 @@ void main() {
 
       debugNetworkImageHttpClientProvider = null;
     });
+
+    testWidgets('options are shown in reversed order', (tester) async {
+      final users = List.generate(8, (i) => eg.user(fullName: 'A$i', avatarUrl: 'user$i.png'));
+      final composeInputFinder = await setupToComposeInput(tester, users: users);
+      final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+
+      // TODO(#226): Remove this extra edit when this bug is fixed.
+      await tester.enterText(composeInputFinder, 'hello @');
+      await tester.enterText(composeInputFinder, 'hello @A');
+      await tester.pump();
+
+      final initialPosition = tester.getTopLeft(find.text(users.first.fullName)).dy;
+      // Initially, all but the last autocomplete options are visible.
+      checkUserShown(users.last, store, expected: false);
+      users.take(7).forEach((user) => checkUserShown(user, store, expected: true));
+
+      // Can't scroll down because the options grow from the bottom.
+      await tester.drag(find.byType(ListView), const Offset(0, -50));
+      await tester.pump();
+      check(tester.getTopLeft(find.text(users.first.fullName)).dy)
+        .equals(initialPosition);
+
+      // The last autocomplete option becomes visible after scrolling up.
+      await tester.drag(find.byType(ListView), const Offset(0, 200));
+      await tester.pump();
+      users.skip(1).forEach((user) => checkUserShown(user, store, expected: true));
+      checkUserShown(users.first, store, expected: false);
+
+      debugNetworkImageHttpClientProvider = null;
+    });
   });
 
   group('emoji', () {
@@ -247,6 +277,68 @@ void main() {
       debugNetworkImageHttpClientProvider = null;
     });
 
+ testWidgets('emoji options appear in the correct rendering order and do not scroll down', (tester) async {
+    final composeInputFinder = await setupToComposeInput(tester);
+    final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+
+    store.setServerEmojiData(
+      ServerEmojiData(
+        codeToNames: {
+          '1f4a4': ['zzz'],
+          '1f52a': ['biohazard'],
+          '1f92a': ['zany_face'],
+          '1f993': ['zebra'],
+          '0030-fe0f-20e3': ['zero'],
+          '1f9d0': ['zombie'],
+          }));
+
+    await store.handleEvent(
+      RealmEmojiUpdateEvent(
+        id: 1,
+        realmEmoji: {
+          '1': eg.realmEmojiItem(emojiCode: '1', emojiName: 'buzzing')}));
+
+    final emojiSequence = ['zulip','zzz','💤','zany_face','zebra','zero','zombie','buzzing','biohazard'];
+
+    // Enter a query; options appear, of all three emoji types.
+    // TODO(#226): Remove this extra edit when this bug is fixed.
+    await tester.enterText(composeInputFinder, 'hi :');
+    await tester.enterText(composeInputFinder, 'hi :z');
+    await tester.pump();
+
+    final positions = emojiSequence.take(8).map((icon) {
+      final finder = find.text(icon);
+      check(because:"Each emoji option should be rendered", finder).findsOne();
+      return tester.getTopLeft(finder).dy;
+    }).toList();
+
+    await tester.drag(find.byType(ListView), const Offset(0, -50));
+    await tester.pump();
+    final firstEmojiPositionAfterScrollDown = tester.getTopLeft(find.text(emojiSequence[0])).dy;
+    check(
+      because:"ListView options should not scroll down further than initial position",
+      positions.first
+    ).equals(firstEmojiPositionAfterScrollDown);
+
+    check(
+      because: "The last emoji should not be visible before scrolling up"
+      ,find.text(emojiSequence.last)
+    ).findsNothing();
+
+    // Scroll up
+    await tester.drag(find.byType(ListView), const Offset(0, 50));
+    await tester.pump();
+
+    check(because:"The last emoji should be visible after scrolling up",
+    find.text(emojiSequence.last)).findsOne();
+
+    final firstEmojiPositionAfterScrollUp = tester.getTopLeft(find.text(emojiSequence[0])).dy;
+    check(because: "Scrolling up should reveal other emoji matches",firstEmojiPositionAfterScrollUp)
+    .isGreaterOrEqual(positions.first);
+
+    debugNetworkImageHttpClientProvider = null;
+  });
+
     testWidgets('text emoji means just show text', (tester) async {
       final composeInputFinder = await setupToComposeInput(tester);
       final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);