compose_box [nfc]: Avoid inline checks on narrows.

This will later help ensure the exhaustiveness of similar checks
performed on a Narrow. The change is motivated by the need to support
new narrows.

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

Author: PIG208

lib/widgets/compose_box.dart

@@ -958,6 +958,18 @@ class ComposeBox extends StatelessWidget {
   final GlobalKey<ComposeBoxController>? controllerKey;
   final Narrow narrow;
 
+  static bool hasComposeBox(Narrow narrow) {
+    switch (narrow) {
+      case CombinedFeedNarrow():
+        return true;
+
+      case StreamNarrow():
+      case TopicNarrow():
+      case DmNarrow():
+        return false;
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     final narrow = this.narrow;

lib/widgets/message_list.dart

@@ -130,7 +130,7 @@ class _MessageListPageState extends State<MessageListPage> implements MessageLis
               //   if those details get complicated, refactor to avoid copying.
               // TODO(#311) If we have a bottom nav, it will pad the bottom
               //   inset, and this should always be true.
-              removeBottom: widget.narrow is! CombinedFeedNarrow,
+              removeBottom: !ComposeBox.hasComposeBox(widget.narrow),
 
               child: Expanded(
                 child: MessageList(narrow: widget.narrow))),
@@ -361,7 +361,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
           return _buildItem(data, i);
         }));
 
-    if (widget.narrow is CombinedFeedNarrow) {
+    if (ComposeBox.hasComposeBox(widget.narrow)) {
       // TODO(#311) If we have a bottom nav, it will pad the bottom
       //   inset, and this shouldn't be necessary
       sliver = SliverSafeArea(sliver: sliver);
@@ -542,7 +542,7 @@ class RecipientHeader extends StatelessWidget {
     final message = this.message;
     return switch (message) {
       StreamMessage() => StreamMessageRecipientHeader(message: message,
-        showStream: narrow is CombinedFeedNarrow),
+        showStream: ComposeBox.hasComposeBox(narrow)),
       DmMessage() => DmRecipientHeader(message: message),
     };
   }