compose: Fix bug where compose progress can be lost on new event queue

And factor out a helper for the part that reads `widget.narrow` and
sets a new controller accordingly; we'll use this helper for the
edit-message UI, to reset the compose box at the end of an
edit-message session.

Fixes: zulip#1470

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1523,14 +1523,26 @@ class _ComposeBoxState extends State<ComposeBox> with PerAccountStoreAwareStateM
 
   @override
   void onNewStore() {
+    final newStore = PerAccountStoreWidget.of(context);
+
+    if (_controller == null) {
+      _setNewControllerForNarrow(newStore);
+    } else {
+      switch (_controller) {
+        case StreamComposeBoxController():
+          (_controller as StreamComposeBoxController).topic.store = newStore;
+        case FixedDestinationComposeBoxController():
+          break; // no reference to the store that needs updating
+        case null:
+          assert(false);
+      }
+    }
+  }
+
+  void _setNewControllerForNarrow(PerAccountStore store) {
     switch (widget.narrow) {
       case ChannelNarrow():
-        final store = PerAccountStoreWidget.of(context);
-        if (_controller == null) {
-          _controller = StreamComposeBoxController(store: store);
-        } else {
-          (controller as StreamComposeBoxController).topic.store = store;
-        }
+        _controller = StreamComposeBoxController(store: store);
       case TopicNarrow():
       case DmNarrow():
         _controller = FixedDestinationComposeBoxController();

test/widgets/compose_box_test.dart

@@ -56,7 +56,8 @@ void main() {
     addTearDown(testBinding.reset);
     selfUser ??= eg.selfUser;
     zulipFeatureLevel ??= eg.futureZulipFeatureLevel;
-    final selfAccount = eg.account(user: selfUser, zulipFeatureLevel: zulipFeatureLevel);
+    final selfAccount = eg.account(
+      id: eg.selfAccount.id, user: selfUser, zulipFeatureLevel: zulipFeatureLevel);
     await testBinding.globalStore.add(selfAccount, eg.initialSnapshot(
       zulipFeatureLevel: zulipFeatureLevel,
       realmMandatoryTopics: mandatoryTopics,
@@ -111,6 +112,11 @@ void main() {
     await tester.enterText(contentInputFinder, content);
   }
 
+  void checkContentInputValue(WidgetTester tester, String expected) {
+    check(tester.widget<TextField>(contentInputFinder))
+      .controller.isNotNull().value.text.equals(expected);
+  }
+
   Future<void> tapSendButton(WidgetTester tester) async {
     connection.prepare(json: SendMessageResult(id: 123).toJson());
     await tester.tap(find.byIcon(ZulipIcons.send));
@@ -1239,4 +1245,41 @@ void main() {
         maxHeight: verticalPadding + 170 * 1.5, maxVisibleLines: 6);
     });
   });
+
+  group('ComposeBoxState new-event-queue transition', () {
+    testWidgets('content input not cleared when store changes', (tester) async {
+      // Regression test for: https://github.com/zulip/zulip-flutter/issues/1470
+
+      TypingNotifier.debugEnable = false;
+      addTearDown(TypingNotifier.debugReset);
+
+      final channel = eg.stream();
+      await prepareComposeBox(tester,
+        narrow: eg.topicNarrow(channel.streamId, 'topic'), streams: [channel]);
+
+      await enterContent(tester, 'some content');
+      checkContentInputValue(tester, 'some content');
+
+      store.updateMachine!
+        ..debugPauseLoop()
+        ..poll()
+        ..debugPrepareLoopError(
+            eg.apiExceptionBadEventQueueId(queueId: store.queueId))
+        ..debugAdvanceLoop();
+      await tester.pump();
+
+      // new store replaces the old one
+      final newStore = testBinding.globalStore.perAccountSync(eg.selfAccount.id)!;
+      check(store).not((it) => it.identicalTo(newStore));
+
+      // new store has the same basic boring data so the compose box
+      // has its content input instead of no-posting-permission error banner
+      // TODO instead, add data by preparing new store's initial snapshot
+      await newStore.addStream(channel);
+      await newStore.addUser(eg.selfUser);
+      await tester.pump();
+
+      checkContentInputValue(tester, 'some content');
+    });
+  });
 }