compose_box: Send "typing stopped" notices on app lifecycle updates

This is a bonus feature that covers some cases that FocusNode doesn't
cover.  We send a "typing stopped" notice when the app loses focus or
becomes invisible.

An example of WidgetsBindingObserver can be found here:
  https://api.flutter.dev/flutter/widgets/WidgetsBindingObserver-class.html

The cited text comes from the AppLifecycleState documentation:
  https://github.com/flutter/engine/blob/a65f1d59edc618ae81e2e8ed78d59fb729291afa/lib/ui/platform_dispatcher.dart#L1856-L1991

The link is not included in code because the code themselves are
references to their documentation already.

Signed-off-by: Zixuan James Li <[email protected]>

lib/widgets/compose_box.dart

@@ -288,12 +288,13 @@ class _ContentInput extends StatefulWidget {
   State<_ContentInput> createState() => _ContentInputState();
 }
 
-class _ContentInputState extends State<_ContentInput> {
+class _ContentInputState extends State<_ContentInput> with WidgetsBindingObserver {
   @override
   void initState() {
     super.initState();
     widget.controller.addListener(_contentChanged);
     widget.focusNode.addListener(_focusChanged);
+    WidgetsBinding.instance.addObserver(this);
   }
 
   @override
@@ -313,6 +314,7 @@ class _ContentInputState extends State<_ContentInput> {
   void dispose() {
     widget.controller.removeListener(_contentChanged);
     widget.focusNode.removeListener(_focusChanged);
+    WidgetsBinding.instance.removeObserver(this);
     super.dispose();
   }
 
@@ -333,6 +335,34 @@ class _ContentInputState extends State<_ContentInput> {
     store.typingNotifier.stoppedComposing();
   }
 
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    switch (state) {
+      case AppLifecycleState.hidden:
+      case AppLifecycleState.paused:
+      case AppLifecycleState.detached:
+        // Transition to either [hidden] or [paused] signals that
+        // > [the] application is not currently visible to the user, and not
+        // > responding to user input.
+        //
+        // When transitioning to [detached], the compose box can't exist:
+        // > The application defaults to this state before it initializes, and
+        // > can be in this state (applicable on Android, iOS, and web) after
+        // > all views have been detached.
+        //
+        // For all these states, we can conclude that the user is not
+        // composing a message.
+        final store = PerAccountStoreWidget.of(context);
+        store.typingNotifier.stoppedComposing();
+      case AppLifecycleState.inactive:
+        // > At least one view of the application is visible, but none have
+        // > input focus. The application is otherwise running normally.
+        // For example, we expect this state when the user is selecting a file
+        // to upload.
+      case AppLifecycleState.resumed:
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     ColorScheme colorScheme = Theme.of(context).colorScheme;

test/widgets/compose_box_test.dart

@@ -344,6 +344,28 @@ void main() {
       await tester.pump(store.typingNotifier.typingStoppedWaitPeriod);
       checkTypingRequest(TypingOp.stop, narrow);
     });
+
+    testWidgets('unfocusing app sends a "typing stopped" notice', (tester) async {
+      await prepareComposeBox(tester, narrow: narrow);
+
+      await checkStartTyping(tester, narrow);
+
+      connection.prepare(json: {});
+      // While this state lives on [ServicesBinding], testWidgets resets it
+      // for us when the test ends so we don't have to:
+      //   https://github.com/flutter/flutter/blob/c78c166e3ecf963ca29ed503e710fd3c71eda5c9/packages/flutter_test/lib/src/binding.dart#L1189
+      // On iOS and Android, a transition to [hidden] is synthesized before
+      // transitioning into [paused].
+      WidgetsBinding.instance.handleAppLifecycleStateChanged(
+        AppLifecycleState.hidden);
+      await tester.pump(Duration.zero);
+      checkTypingRequest(TypingOp.stop, narrow);
+
+      WidgetsBinding.instance.handleAppLifecycleStateChanged(
+        AppLifecycleState.paused);
+      await tester.pump(Duration.zero);
+      check(connection.lastRequest).isNull();
+    });
   });
 
   group('message-send request response', () {