compose: In topic or DM narrow, compose progress is lost when event queue expires

[chrisbobbe]

To reproduce:

1. Open a DM narrow or a topic narrow
2. Type some text into the compose box
3. Put the app in the background and wait at least 10 minutes
4. Foreground the app; wait for new-queue loading indicator to disappear

Expected behavior: The text in the compose box is still there

Actual behavior: The text is cleared

In f1646c4, _ComposeBoxState started using PerAccountStoreAwareStateMixin. For DM and topic narrows, its onNewStore implementation drops the FixedDestinationComposeBoxController on the floor (without calling dispose) and makes a new one. It should only make a new one if it's the first time it's called.

Code:

  @override
  void onNewStore() {
    switch (widget.narrow) {
      case ChannelNarrow():
        final store = PerAccountStoreWidget.of(context);
        if (_controller == null) {
          _controller = StreamComposeBoxController(store: store);
        } else {
          (controller as StreamComposeBoxController).topic.store = store;
        }
      case TopicNarrow():
      case DmNarrow():
        _controller = FixedDestinationComposeBoxController();
      case CombinedFeedNarrow():
      case MentionsNarrow():
      case StarredMessagesNarrow():
        assert(false);
    }
  }

Possible fix (untested):

diff --git lib/widgets/compose_box.dart lib/widgets/compose_box.dart
index 966bec5f2..8d4b5fccc 100644
--- lib/widgets/compose_box.dart
+++ lib/widgets/compose_box.dart
@@ -1533,7 +1533,7 @@ class _ComposeBoxState extends State<ComposeBox> with PerAccountStoreAwareStateM
         }
       case TopicNarrow():
       case DmNarrow():
-        _controller = FixedDestinationComposeBoxController();
+        _controller ??= FixedDestinationComposeBoxController();
       case CombinedFeedNarrow():
       case MentionsNarrow():
       case StarredMessagesNarrow():

Assigning @PIG208 as the author of f1646c4.

[chrisbobbe]

I'm refactoring this code on the way to #126, so I'll take this issue as part of that work. Edit: sent #1482