To improve usability I have removed the notion of an "active part list", and have also changed how saved and completed builds are updated.
Starting now, any changes made to a part list will not automatically update a saved or completed build. Instead, to update a saved or completed build, use the part list "Save As" button and select the build you would like to update. You can also save your part list to a new saved or completed build.
Before this update, you could switch the active part list to a previously saved or completed build. Then any changes made would be automatically be reflected in your saved or completed build.
In retrospect this was not a good design decision. I tried to make the currently active part list obvious, but in the end no matter what I did I couldn't make it visible enough. Changes to saved and completed builds were made accidentally because it wasn't clear that they were the active part list. I received a lot of feedback to this effect, so I sought out to fix things.
To remedy this problem, I reverted the part list editing behavior to be similar to how to it was prior to the previous changes. Changes made to a part list no longer automatically update a saved or completed build. Now if you want to update a previously saved or completed build, you can use the "Save As" button to either select a build and overwrite it, or create an entirely new saved/completed build.
This will hopefully prevent accidental changes to saved and completed builds. I apologize to anyone affected aversely by the previous strategy, and I appreciate your patience with me and the site as I find the best approach. I hope that this new system is safer, and in the end, more usable. As always, if you have feedback or suggestions, definitely let me know.