I do not know the back story on why the Completion Insight feature is no longer automatically "on" for the object viewer's Data tab. It may be a bug, or it may have been intentional. The Tools > Preferences... > Code Editor > Completion Insight settings seem to have no effect.
Fortunately the shortcut key for Completion Insight, Ctrl+Space, either before or after typing something in the Filter input area, works fine.
SQL Developer Team
I had the 'automatic' bit of the feature removed - while typing a column name, the pop-up would frequently get in the way, and it was very hard to select something w/o the already-typed in text not getting corrupted. But like Gary said, you can still get the help, you just have to ask for it.