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 behavior rendering behavior.

Fixes zulip#1123.
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

@@ -247,6 +247,101 @@ 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', 'sleepy'], // Unicode emoji for "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'),
+        },
+      ),
+    );
+
+    const zulipOptionLabel = 'zulip';
+    const zanyFaceOptionLabel = 'zany_face';
+    const zebraOptionLabel = 'zebra';
+    const zzzOptionLabel = 'zzz, sleepy';
+    const unicodeGlyph = '💤';
+    const zombieOptionLabel = 'zombie';
+    const zeroOptionLabel = 'zero';
+    const buzzingOptionLabel = 'buzzing';
+    const biohazardOptionLabel = 'biohazard';
+
+    // Adjust the order so the best match appears last
+    final emojiSequence = [
+      zulipOptionLabel,
+      zzzOptionLabel,
+      unicodeGlyph,
+      zanyFaceOptionLabel,
+      zebraOptionLabel,
+      zeroOptionLabel,
+      zombieOptionLabel,
+      buzzingOptionLabel,
+      // biohazardOptionLabel, this won't be rendered in the list initally since it is the 7th option.
+    ];
+
+    await tester.enterText(composeInputFinder, 'hi :');
+    await tester.enterText(composeInputFinder, 'hi :z');
+    await tester.pumpAndSettle();
+
+    final listViewFinder = find.byType(ListView);
+    expect(listViewFinder, findsOneWidget, reason: 'ListView should be rendered');
+
+    final positions = emojiSequence.map((icon) {
+      final finder = find.text(icon);
+      expect(finder, findsOneWidget, reason: 'Each emoji option should be rendered');
+      return tester.getTopLeft(finder).dy;
+    }).toList();
+
+    for (int i = 0; i < positions.length - 1; i++) {
+      expect(positions[i] > positions[i + 1], isTrue,
+          reason: '${emojiSequence[i + 1]} should appear above ${emojiSequence[i]} because of reverse order');
+    }
+
+    final initialScrollOffset = tester.getTopLeft(listViewFinder).dy;
+    await tester.drag(listViewFinder, const Offset(0, -50));
+    await tester.pumpAndSettle();
+    final scrollOffsetAfterDragDown = tester.getTopLeft(listViewFinder).dy;
+
+    expect(scrollOffsetAfterDragDown, initialScrollOffset,
+        reason: 'ListView should not scroll down because it is already at the bottom');
+
+    final biohazardFinder = find.text(biohazardOptionLabel);
+    expect(biohazardFinder, findsNothing, reason: 'The biohazard emoji should not be visible before scrolling up');
+
+    // Scroll up
+    await tester.drag(listViewFinder, const Offset(0, 50));
+    await tester.pumpAndSettle();
+
+    expect(biohazardFinder, findsOneWidget, reason: 'The biohazard emoji should be visible after scrolling up');
+
+    final firstEmojiPositionAfterScrollUp = tester.getTopLeft(find.text(emojiSequence[0])).dy;
+    // print(firstEmojiPositionAfterScrollUp);
+    // print(positions[0]);
+    expect(firstEmojiPositionAfterScrollUp >= positions[0], isTrue,
+        reason: 'Scrolling up should reveal other emoji matches');
+
+    expect(tester.getTopLeft(find.text(biohazardOptionLabel)).runtimeType,const Offset(1,2).runtimeType, reason:"7th best result should only be visible on scrolling up");
+
+    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);