I have an Xcode 12.3 XIB project using Objective C running on Catalina with a panel to which I have added a vertical split view of type NSSplitview using default attributes. The split view divider is not visible when I run the application, but objects placed in each pane of the NSSplitview are. The invisible divider (there is no change in the cursor when moving over where the divider should be) means that NSSplitview does not work.
If a create a new XIB project, an NSSplitview control added to a panel is visible OK, so I think there is something about the parent project which is upsetting NSSplitview.