This article will show you how to fix the broken structure panel in Oxygen Builder version 3.9. Due to a structural change in this release, previous CSS no longer applies to the new version of the structure panel. In our opinion, this is a welcome change as it makes it easier to drag and drop various elements within it, which was a problem in previous versions.
However, due to old CSS being applied to new HTML structures within the structure panel, it may appear broken when updating from version 3.8 to 3.9.
Luckily, it's a quick fix.
If it's not, another thing to try would be removing the cookies for the website or page. While this process difference per browser, you can typically go into the storage tab of your developer tools, select all cookies and local storage, and delete them.
One final thing to take note of is that the developers intend to add better cash busting CSS versioning to future versions of this builder. This was not caused by user error, but was an oversight by the development team. Luckily, enough people had an issue with it that it is incredibly likely that we won't see a problem like this again.