If I understand you correctly you want to test a new feature out and then remove that feature from the project 
Regardless exiting the designer keeps the project as it was last saved, typically this is the last saved staging version of the project (depending on how you have it configured).
So if you haven’t saved anything when using the designer then yes it will revert to the existing project. 
In my work flow I typically test out features that are not going to be implemented in a cloned project.