msglist [nfc]: Simplify safe-area handling

This addresses Greg's TODO in 88f3b73:

  // TODO have MessageListPage adjust the MediaQuery so we don't
  //   have to worry about this here

Not seeing (experimentally) any more bugs like the one fixed in the
previous commit, I'm marking this NFC.

Author: chrisbobbe

lib/widgets/app.dart

@@ -150,9 +150,23 @@ class MessageListPage extends StatelessWidget {
     return Scaffold(
         appBar: AppBar(title: const Text("All messages")),
         body: Center(
-            child: Column(children: const [
-          Expanded(child: MessageList()),
-          SizedBox(
+            child: Column(children: [
+          MediaQuery.removePadding(
+              context: context,
+
+              // The app bar pads the top inset. (If `context` were for a
+              // location under Scaffold's `body`, we wouldn't have to
+              // `removeTop`. Scaffold, which knows about the app bar,
+              // already applies `removeTop` on `body`:
+              //   https://github.com/flutter/flutter/blob/3.7.0-1.2.pre/packages/flutter/lib/src/material/scaffold.dart#L2778
+              // )
+              removeTop: true,
+
+              // The compose box pads the bottom inset.
+              removeBottom: true,
+
+              child: const Expanded(child: MessageList())),
+          const SizedBox(
               height: 80,
               child: Center(child: Text("(Compose box goes here.)"))),
         ])));

lib/widgets/message_list.dart

@@ -68,18 +68,6 @@ class _MessageListState extends State<MessageList> {
 
             // Pad the left and right insets, for small devices in landscape.
             child: SafeArea(
-                // A non-ancestor (the compose box) pads the bottom inset;
-                // prevent extra padding here.
-                // TODO have MessageListPage adjust the MediaQuery so we don't
-                //   have to worry about this here
-                bottom: false,
-
-                // A non-ancestor (the app bar) pads the top inset. But no
-                // need to prevent extra padding with `top: false`, because
-                // Scaffold, which knows about the app bar, sets `body`'s
-                // ambient `MediaQueryData.padding` to have `top: 0`:
-                //   https://github.com/flutter/flutter/blob/3.7.0-1.2.pre/packages/flutter/lib/src/material/scaffold.dart#L2778
-
                 // Keep some padding when there are no horizontal insets,
                 // which is usual in portrait mode.
                 minimum: const EdgeInsets.symmetric(horizontal: 8),
@@ -93,24 +81,6 @@ class _MessageListState extends State<MessageList> {
     final length = model!.messages.length;
     assert(model!.contents.length == length);
     return StickyHeaderListView.builder(
-        // If the `padding` param is omitted or null, StickyHeaderListView pads
-        // the content area (the part that moves when you scroll) with
-        // `MediaQueryData.padding`. It inherits this behavior from
-        // BoxScrollView:
-        //   https://github.com/flutter/flutter/blob/3.7.0-1.2.pre/packages/flutter/lib/src/widgets/scroll_view.dart#L665-L687
-        //
-        // A non-ancestor (the compose box) pads the bottom inset, so the
-        // ambient `MediaQueryData.padding` might be nonzero. So, prevent double
-        // padding at the bottom by supplying a `padding` param.
-        //
-        // (A non-ancestor, the app bar, pads the top inset, but we don't
-        // need the remedy of a non-null `padding` for the top because the
-        // ambient `MediaQueryData.padding` has `top: 0` thanks to Scaffold
-        // setting that on its `body`, since it knows about the app bar:
-        //   https://github.com/flutter/flutter/blob/3.7.0-1.2.pre/packages/flutter/lib/src/material/scaffold.dart#L2778
-        // )
-        padding: const EdgeInsets.all(0),
-
         itemCount: length,
         // Setting reverse: true means the scroll starts at the bottom.
         // Flipping the indexes (in itemBuilder) means the start/bottom