Use actions to fix showing handler

Author: singerdmx

lib/src/widgets/editor.dart

@@ -1544,13 +1544,50 @@ class RenderEditor extends RenderEditableContainerBox
 
   // End TextLayoutMetrics implementation
 
+  QuillVerticalCaretMovementRun startVerticalCaretMovement(
+      TextPosition startPosition) {
+    return QuillVerticalCaretMovementRun._(
+      this,
+      startPosition,
+    );
+  }
+
   @override
   void systemFontsDidChange() {
     super.systemFontsDidChange();
     markNeedsLayout();
   }
 }
 
+class QuillVerticalCaretMovementRun
+    extends BidirectionalIterator<TextPosition> {
+  QuillVerticalCaretMovementRun._(
+    this._editor,
+    this._currentTextPosition,
+  );
+
+  TextPosition _currentTextPosition;
+
+  final RenderEditor _editor;
+
+  @override
+  TextPosition get current {
+    return _currentTextPosition;
+  }
+
+  @override
+  bool moveNext() {
+    _currentTextPosition = _editor.getTextPositionBelow(_currentTextPosition);
+    return true;
+  }
+
+  @override
+  bool movePrevious() {
+    _currentTextPosition = _editor.getTextPositionAbove(_currentTextPosition);
+    return true;
+  }
+}
+
 class EditableContainerParentData
     extends ContainerBoxParentData<RenderEditableBox> {}