Completely delete that directory and re-install it. Bad things happen when you extract a copy of sql developer into an existing installation.
Sorry, I may not have been clear. I did remove the existing directly entirely before doing the install. That is, I renamed that directory. Then created a new directory with the same name. So, both version should are still distinct.
If the folder is called "SQL Developer", I don't have the "Oracle" tab. If I rename the folder to something else, then run SQL Developer, the "Oracle" tab is present. If I then change the folder name back, the tab is again gone. The contents of the folder rename unchanged each time I try this.