A new feature in the latest release of CPQ is the ability to collapse a bundle to rescue screen real estate. This is a great feature since so many bundles have lots of quote lines and not necessary to see all the details every time.
This is Salesforce help article that describes it.
What’s interesting about this article however is that Large Quote Experience (LQE) must be on for this feature to work. Lots of configurations are not using this feature and it may crash yours. Look at your validation product rules and see if they use Summary Variables. You may need to rethink these as LQE borks those rules….
Still — a useful UI feature.