fix: #1467 Editor's left and right margins is too rigid (#1471)

frontend/app_flowy/lib/plugins/document/document_page.dart

@@ -96,6 +96,7 @@ class _DocumentPageState extends State<DocumentPage> {
 
   Widget _renderAppFlowyEditor(EditorState editorState) {
     final theme = Theme.of(context);
+    final editorMaxWidth = MediaQuery.of(context).size.width * 0.6;
     final editor = AppFlowyEditor(
       editorState: editorState,
       autoFocus: editorState.document.isEmpty,
@@ -112,8 +113,13 @@ class _DocumentPageState extends State<DocumentPage> {
       ]),
     );
     return Expanded(
-      child: SizedBox.expand(
-        child: editor,
+      child: Center(
+        child: Container(
+          constraints: BoxConstraints(
+            maxWidth: editorMaxWidth,
+          ),
+          child: editor,
+        ),
       ),
     );
   }

frontend/app_flowy/lib/plugins/document/editor_styles.dart

@@ -8,6 +8,7 @@ EditorStyle customEditorTheme(BuildContext context) {
       ? EditorStyle.dark
       : EditorStyle.light;
   editorStyle = editorStyle.copyWith(
+    padding: const EdgeInsets.all(0),
     textStyle: editorStyle.textStyle?.copyWith(
       fontFamily: 'poppins',
       fontSize: _baseFontSize,