Program: 123515
Project: 123515
Buggy source:
private boolean checkDropIsValid(){
  if (objects == null || objects.getToTransfer().isEmpty() || targetEditPart == null) {
    return false;
  }
  Object targetModel=targetEditPart.getModel();
  targetFigure=targetEditPart.getFigure().findFigureAt(dropLocation.x,dropLocation.y);
  System.out.println(targetFigure);
  boolean isTragetOutputNode=targetModel instanceof OutputTreeNode;
  boolean isTragetInputNode=targetModel instanceof TreeNode && !(targetModel instanceof OutputTreeNode);
  boolean isVar=targetModel instanceof VarNode;
  boolean isDropInputTree=targetModel instanceof InputXmlTree;
  boolean isDropOutputTree=targetModel instanceof OutputXmlTree;
  boolean isDropVarTable=targetModel instanceof VarTable;
  if (targetFigure instanceof ExpressionFigure) {
    dropType=DropType.DROP_EXPRESSION;
    if (isTragetOutputNode) {
      return XmlMapUtil.isExpressionEditable((TreeNode)targetModel);
    }
 else     if (isTragetInputNode) {
      boolean isValid=checkDropInputValid(targetModel);
      if (isValid) {
        isValid=XmlMapUtil.isExpressionEditable((TreeNode)targetModel);
      }
      return isValid;
    }
    return true;
  }
 else   if (targetFigure instanceof FilterTextArea) {
    dropType=DropType.DROP_FILTER;
    return true;
  }
 else {
    if (objects.getType() == TransferdType.INPUT) {
      if (isTragetOutputNode) {
        return checkDropOutputDocChildValid(targetModel);
      }
 else       if (isVar || isDropVarTable) {
        dropType=DropType.DROP_INSERT_VAR;
        return true;
      }
 else       if (isDropInputTree) {
        dropType=DropType.DROP_INSERT_INPUT;
        return checkDropInputValid(targetModel);
      }
 else       if (isDropOutputTree) {
        dropType=DropType.DROP_INSERT_OUTPUT;
        return true;
      }
    }
 else     if (objects.getType() == TransferdType.VAR) {
      if (isTragetOutputNode) {
        return checkDropOutputDocChildValid(targetModel);
      }
 else       if (isDropOutputTree) {
        dropType=DropType.DROP_INSERT_OUTPUT;
        return true;
      }
    }
  }
  return false;
}

Fixed version (ground truth):
private boolean checkDropIsValid(){
  if (objects == null || objects.getToTransfer().isEmpty() || targetEditPart == null) {
    return false;
  }
  Object targetModel=targetEditPart.getModel();
  targetFigure=targetEditPart.getFigure().findFigureAt(dropLocation.x,dropLocation.y);
  boolean isTragetOutputNode=targetModel instanceof OutputTreeNode;
  boolean isTragetInputNode=targetModel instanceof TreeNode && !(targetModel instanceof OutputTreeNode);
  boolean isVar=targetModel instanceof VarNode;
  boolean isDropInputTree=targetModel instanceof InputXmlTree;
  boolean isDropOutputTree=targetModel instanceof OutputXmlTree;
  boolean isDropVarTable=targetModel instanceof VarTable;
  if (targetFigure instanceof ExpressionFigure) {
    dropType=DropType.DROP_EXPRESSION;
    if (isTragetOutputNode) {
      return XmlMapUtil.isExpressionEditable((TreeNode)targetModel);
    }
 else     if (isTragetInputNode) {
      boolean isValid=checkDropInputValid(targetModel);
      if (isValid) {
        isValid=XmlMapUtil.isExpressionEditable((TreeNode)targetModel);
      }
      return isValid;
    }
    return true;
  }
 else   if (targetFigure instanceof FilterTextArea) {
    dropType=DropType.DROP_FILTER;
    return true;
  }
 else {
    if (objects.getType() == TransferdType.INPUT) {
      if (isTragetOutputNode) {
        return checkDropOutputDocChildValid(targetModel);
      }
 else       if (isVar || isDropVarTable) {
        dropType=DropType.DROP_INSERT_VAR;
        return true;
      }
 else       if (isDropInputTree) {
        dropType=DropType.DROP_INSERT_INPUT;
        return checkDropInputValid(targetModel);
      }
 else       if (isDropOutputTree) {
        dropType=DropType.DROP_INSERT_OUTPUT;
        return true;
      }
    }
 else     if (objects.getType() == TransferdType.VAR) {
      if (isTragetOutputNode) {
        return checkDropOutputDocChildValid(targetModel);
      }
 else       if (isDropOutputTree) {
        dropType=DropType.DROP_INSERT_OUTPUT;
        return true;
      }
    }
  }
  return false;
}