Preserve inline style when replacing formatted text from the first character (#201)

packages/notus/lib/src/document.dart

     // We have to compose before applying delete rules
     // Otherwise delete would be operating on stale document snapshot.
     if (text.isNotEmpty) {
-      delta = insert(index, text);
-      index = delta.transformPosition(index);
+      delta = insert(index + length, text);
     }
 
     if (length > 0) {

packages/zefyr/lib/src/widgets/controller.dart

       // some style, then we apply it to our document.
       if (delta != null &&
           toggledStyles.isNotEmpty &&
-          delta.length == 2 &&
-          delta[1].isInsert) {
+          delta.length <=2 &&
+          delta.last.isInsert) {
         // Apply it.
         Delta retainDelta = Delta()
           ..retain(index)
-          ..retain(1, toggledStyles.toJson());
+          ..retain(text.length, toggledStyles.toJson());
         document.compose(retainDelta, ChangeSource.local);
       }
     }

packages/zefyr/test/widgets/controller_test.dart

       var result = controller.getSelectionStyle();
       expect(result.values, [NotusAttribute.bold]);
     });
+    test('formatText with toggled style for first charater', () {
+      bool notified = false;
+      controller.addListener(() {
+        notified = true;
+      });
+      controller.formatText(0, 0, NotusAttribute.bold);
+      controller.replaceText(0, 0, 'Word');
+      expect(notified, isTrue);
+      expect(
+        controller.document.toDelta(),
+        Delta()..insert('Word', NotusAttribute.bold.toJson())..insert('\n'),
+      );
+      expect(controller.lastChangeSource, ChangeSource.local);
+    });
   });
 }