msglist [nfc]: Remove one Center widget (because no-op); comment to explain another

lib/widgets/compose_box.dart

@@ -1086,7 +1086,7 @@ class _ComposeBoxContainer extends StatelessWidget {
     };
 
     // TODO(design): Maybe put a max width on the compose box, like we do on
-    //   the message list itself
+    //   the message list itself; if so, remember to update ComposeBox's dartdoc.
     return Container(width: double.infinity,
       decoration: BoxDecoration(
         border: Border(top: BorderSide(color: designVariables.borderBar))),
@@ -1275,6 +1275,10 @@ class _ErrorBanner extends StatelessWidget {
   }
 }
 
+/// The compose box.
+///
+/// Takes the full screen width, covering the horizontal insets with its surface.
+/// Also covers the bottom inset with its surface.
 class ComposeBox extends StatefulWidget {
   ComposeBox({super.key, required this.narrow})
     : assert(ComposeBox.hasComposeBox(narrow));

lib/widgets/message_list.dart

@@ -294,8 +294,11 @@ class _MessageListPageState extends State<MessageListPage> implements MessageLis
       //   we matched to the Figma in 21dbae120. See another frame, which uses that:
       //     https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=147%3A9088&mode=dev
       body: Builder(
-        builder: (BuildContext context) => Center(
-          child: Column(children: [
+        builder: (BuildContext context) => Column(
+          // Children are expected to take the full horizontal space
+          // and handle the horizontal device insets.
+          // The bottom inset should be handled by the last child only.
+          children: [
             MediaQuery.removePadding(
               // Scaffold knows about the app bar, and so has run this
               // BuildContext, which is under `body`, through
@@ -315,7 +318,7 @@ class _MessageListPageState extends State<MessageListPage> implements MessageLis
                 ))),
             if (ComposeBox.hasComposeBox(narrow))
               ComposeBox(key: _composeBoxKey, narrow: narrow)
-          ]))));
+          ])));
   }
 }
 
@@ -432,6 +435,12 @@ const _kShortMessageHeight = 80;
 // previous batch.
 const kFetchMessagesBufferPixels = (kMessageListFetchBatchSize / 2) * _kShortMessageHeight;
 
+/// The message list.
+///
+/// Takes the full screen width, keeping its contents
+/// out of the horizontal insets with transparent [SafeArea] padding.
+/// When there is no [ComposeBox], also takes responsibility
+/// for dealing with the bottom inset.
 class MessageList extends StatefulWidget {
   const MessageList({super.key, required this.narrow, required this.onNarrowChanged});
 
@@ -525,12 +534,15 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
     // Pad the left and right insets, for small devices in landscape.
     return SafeArea(
       // Don't let this be the place we pad the bottom inset. When there's
-      // no compose box, we want to let the message-list content pad it.
+      // no compose box, we want to let the message-list content
+      // and the scroll-to-bottom button avoid it.
       // TODO(#311) Remove as unnecessary if we do a bottom nav.
       //   The nav will pad the bottom inset, and an ancestor of this widget
       //   will have a `MediaQuery.removePadding` with `removeBottom: true`.
       bottom: false,
 
+      // Horizontally, on wide screens, this Center grows the SafeArea
+      // to position its padding over the device insets and centers content.
       child: Center(
         child: ConstrainedBox(
           constraints: const BoxConstraints(maxWidth: 760),
@@ -543,7 +555,9 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
                   bottom: 0,
                   right: 0,
                   // TODO(#311) SafeArea shouldn't be needed if we have a
-                  //   bottom nav. That will pad the bottom inset.
+                  //   bottom nav; that will pad the bottom inset. Remove it,
+                  //   and the mention of bottom-inset handling in
+                  //   MessageList's dartdoc.
                   child: SafeArea(
                     child: ScrollToBottomButton(
                       scrollController: scrollController,
@@ -594,8 +608,8 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
         }));
 
     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
+      // TODO(#311) If we have a bottom nav, it will pad the bottom inset,
+      //   and this can be removed; also remove mention in MessageList dartdoc
       sliver = SliverSafeArea(sliver: sliver);
     }