Make the fn3 EditableText expand to fill the width of its parent

packages/flutter/lib/src/fn3/editable_text.dart

@@ -195,13 +195,16 @@ class EditableTextState extends State<EditableText> {
     else if (!config.focused && _cursorTimer != null)
       _stopCursorTimer();
 
-    return new _EditableTextWidget(
+    return new SizedBox(
+      width: double.INFINITY,
+      child: new _EditableTextWidget(
         value: config.value,
         style: config.style,
         cursorColor: config.cursorColor,
         showCursor: _showCursor,
         onContentSizeChanged: config.onContentSizeChanged,
         scrollOffset: config.scrollOffset
+      )
     );
   }
 }