accounts-ui: Make accounts list scrollable

Before this fix, the list of accounts did not scroll
when they were more than a screenful and would
show an overflow error on the screen.

After this fix, the list of accounts scrolls properly
with no overflow error and without shifting other
content offscreen.

Fixes: #100

lib/widgets/app.dart

@@ -194,12 +194,15 @@ class ChooseAccountPage extends StatelessWidget {
         child: Center(
           child: ConstrainedBox(
             constraints: const BoxConstraints(maxWidth: 400),
-            child: Column(mainAxisAlignment: MainAxisAlignment.center, children: [
-              for (final (:accountId, :account) in globalStore.accountEntries)
-                _buildAccountItem(context,
-                  accountId: accountId,
-                  title: Text(account.realmUrl.toString()),
-                  subtitle: Text(account.email)),
+            child: Column(mainAxisSize: MainAxisSize.min, children: [
+              Flexible(child: SingleChildScrollView(
+                child: Column(mainAxisSize: MainAxisSize.min, children: [
+                  for (final (:accountId, :account) in globalStore.accountEntries)
+                    _buildAccountItem(context,
+                      accountId: accountId,
+                      title: Text(account.realmUrl.toString()),
+                      subtitle: Text(account.email)),
+                ]))),
               const SizedBox(height: 12),
               ElevatedButton(
                 onPressed: () => Navigator.push(context,

test/flutter_checks.dart

@@ -5,6 +5,13 @@ import 'package:checks/checks.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
+extension RectChecks on Subject<Rect> {
+  Subject<double> get top => has((d) => d.top, 'top');
+  Subject<double> get bottom => has((d) => d.bottom, 'bottom');
+
+  // TODO others
+}
+
 extension AnimationChecks<T> on Subject<Animation<T>> {
   Subject<AnimationStatus> get status => has((d) => d.status, 'status');
   Subject<T> get value => has((d) => d.value, 'value');

test/widgets/app_test.dart

@@ -1,11 +1,15 @@
 import 'package:checks/checks.dart';
-import 'package:flutter/widgets.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:zulip/model/database.dart';
 import 'package:zulip/widgets/app.dart';
 import 'package:zulip/widgets/inbox.dart';
 import 'package:zulip/widgets/page.dart';
+import 'package:zulip/widgets/store.dart';
 
 import '../example_data.dart' as eg;
+import '../flutter_checks.dart';
 import '../model/binding.dart';
 import '../test_navigation.dart';
 import 'page_checks.dart';
@@ -51,4 +55,108 @@ void main() {
       ]);
     });
   });
+
+  group('ChooseAccountPage', () {
+    Future<void> setupChooseAccountPage(WidgetTester tester, {
+      required List<Account> accounts,
+    }) async {
+      addTearDown(testBinding.reset);
+
+      for (final account in accounts) {
+        await testBinding.globalStore
+          .insertAccount(account.toCompanion(false));
+      }
+
+      await tester.pumpWidget(
+        const MaterialApp(
+          localizationsDelegates: ZulipLocalizations.localizationsDelegates,
+          supportedLocales: ZulipLocalizations.supportedLocales,
+          home: GlobalStoreWidget(
+            child: ChooseAccountPage())));
+
+      // global store gets loaded
+      await tester.pumpAndSettle();
+    }
+
+    List<Account> generateAccounts(int count) {
+      return List.generate(count, (i) => eg.account(
+        id: i,
+        user: eg.user(fullName: 'User $i', email: 'user$i@example'),
+        apiKey: 'user${i}apikey',
+      ));
+    }
+
+    Finder findAccount(Account account) => find.text(account.email).hitTestable();
+
+    Finder findButton<T extends ButtonStyleButton>({required String withText}) {
+      return find
+        .descendant(of: find.bySubtype<T>(), matching: find.text(withText))
+        .hitTestable();
+    }
+
+    void checkAccountShown(Account account, {required bool expected}) {
+      check(findAccount(account).evaluate()).length.equals(expected ? 1 : 0);
+    }
+
+    void checkButtonShown<T extends ButtonStyleButton>({
+      required String withText,
+      required bool expected,
+    }) {
+      check(findButton<T>(withText: withText).evaluate())
+        .length.equals(expected ? 1 : 0);
+    }
+
+    testWidgets('accounts list is scrollable when more than a screenful', (tester) async {
+      final accounts = generateAccounts(15);
+      await setupChooseAccountPage(tester, accounts: accounts);
+
+      // Accounts list is more than a screenful
+      //  * First account is shown
+      //  * Last account is out of view
+      checkAccountShown(accounts.first, expected: true);
+      checkAccountShown(accounts.last, expected: false);
+
+      // Button to add an account is visible
+      // and not moved offscreen by the long list of accounts
+      checkButtonShown(withText: 'Add an account', expected: true);
+
+      // Accounts list is scrollable to the bottom
+      await tester.scrollUntilVisible(findAccount(accounts.last), 50);
+      checkAccountShown(accounts.last, expected: true);
+    });
+
+    testWidgets('with just one account, the layout is centered', (tester) async {
+      final account = eg.selfAccount;
+      await setupChooseAccountPage(tester, accounts: [account]);
+
+      const buttonText = 'Add an account';
+      checkAccountShown(account, expected: true);
+      checkButtonShown(withText: buttonText, expected: true);
+
+      final screenHeight =
+        (tester.view.physicalSize / tester.view.devicePixelRatio).height;
+
+      check(tester.getRect(findAccount(account)))
+        ..top.isGreaterThan(1 / 3 * screenHeight)
+        ..bottom.isLessThan(2 / 3 * screenHeight);
+
+      check(tester.getRect(findButton(withText: buttonText)))
+        ..top.isGreaterThan(1 / 3 * screenHeight)
+        ..bottom.isLessThan(2 / 3 * screenHeight);
+    });
+
+    testWidgets('with no accounts, the Add an Account button is centered', (tester) async {
+      await setupChooseAccountPage(tester, accounts: []);
+
+      const buttonText = 'Add an account';
+      checkButtonShown(withText: buttonText, expected: true);
+
+      final screenHeight =
+        (tester.view.physicalSize / tester.view.devicePixelRatio).height;
+
+      check(tester.getRect(findButton(withText: buttonText)))
+        ..top.isGreaterThan(1 / 3 * screenHeight)
+        ..bottom.isLessThan(2 / 3 * screenHeight);
+    });
+  });
 }