compose_box [nfc]: Move blockingErrorBanner from _ComposeBoxLayout to caller

This lets _ComposeBoxLayout stay focused on the complexity of how
the text inputs, the editing buttons, and the send button relate
to each other.

In the case where an error banner is present, this also lets us skip
in the build method some logic to compute things that won't end up
getting used.

Author: gnprice

lib/widgets/compose_box.dart

@@ -870,13 +870,11 @@ class _ComposeBoxLayout extends StatelessWidget {
     required this.sendButton,
     required this.contentController,
     required this.contentFocusNode,
-    this.blockingErrorBanner,
   });
 
   final Widget? topicInput;
   final Widget contentInput;
   final Widget sendButton;
-  final Widget? blockingErrorBanner;
   final ComposeContentController contentController;
   final FocusNode contentFocusNode;
 
@@ -900,30 +898,28 @@ class _ComposeBoxLayout extends StatelessWidget {
     );
 
     return _ComposeBoxContainer(
-      child: blockingErrorBanner != null
-        ? SizedBox(width: double.infinity, child: blockingErrorBanner)
-        : Column(children: [
-            Row(crossAxisAlignment: CrossAxisAlignment.end, children: [
-              Expanded(
-                child: Theme(
-                  data: inputThemeData,
-                  child: Column(children: [
-                    if (topicInput != null) topicInput!,
-                    if (topicInput != null) const SizedBox(height: 8),
-                    contentInput,
-                  ]))),
-              const SizedBox(width: 8),
-              sendButton,
-            ]),
-            Theme(
-              data: themeData.copyWith(
-                iconTheme: themeData.iconTheme.copyWith(color: colorScheme.onSurfaceVariant)),
-              child: Row(children: [
-                _AttachFileButton(contentController: contentController, contentFocusNode: contentFocusNode),
-                _AttachMediaButton(contentController: contentController, contentFocusNode: contentFocusNode),
-                _AttachFromCameraButton(contentController: contentController, contentFocusNode: contentFocusNode),
-              ])),
-          ]));
+      child: Column(children: [
+        Row(crossAxisAlignment: CrossAxisAlignment.end, children: [
+          Expanded(
+            child: Theme(
+              data: inputThemeData,
+              child: Column(children: [
+                if (topicInput != null) topicInput!,
+                if (topicInput != null) const SizedBox(height: 8),
+                contentInput,
+              ]))),
+          const SizedBox(width: 8),
+          sendButton,
+        ]),
+        Theme(
+          data: themeData.copyWith(
+            iconTheme: themeData.iconTheme.copyWith(color: colorScheme.onSurfaceVariant)),
+          child: Row(children: [
+            _AttachFileButton(contentController: contentController, contentFocusNode: contentFocusNode),
+            _AttachMediaButton(contentController: contentController, contentFocusNode: contentFocusNode),
+            _AttachFromCameraButton(contentController: contentController, contentFocusNode: contentFocusNode),
+          ])),
+      ]));
   }
 }
 
@@ -1054,6 +1050,13 @@ class _FixedDestinationComposeBoxState extends State<_FixedDestinationComposeBox
 
   @override
   Widget build(BuildContext context) {
+    final errorBanner = _errorBanner(context);
+    if (errorBanner != null) {
+      return _ComposeBoxContainer(
+        child: SizedBox(width: double.infinity,
+          child: errorBanner));
+    }
+
     return _ComposeBoxLayout(
       contentController: _contentController,
       contentFocusNode: _contentFocusNode,
@@ -1067,8 +1070,7 @@ class _FixedDestinationComposeBoxState extends State<_FixedDestinationComposeBox
         topicController: null,
         contentController: _contentController,
         getDestination: () => widget.narrow.destination,
-      ),
-      blockingErrorBanner: _errorBanner(context));
+      ));
   }
 }