I'm working on a Mac app that, on one tester's computer (running El Cap), doesn't respect Spaces. When you switch Spaces, the app moves to the new space unprompted. Is there any AppKit API that could be affecting this?
Well, the tester should verify they haven't accidentally assigned the app to all Spaces. Right-click on app Dock icon, Options, under Assign To, they should either pick a specific Space or None to have it stick to the Space on which it was originally launched.
As for API, yes, if NSWindow.collectionBehavior contains NSWindowCollectionBehaviorCanJoinAllSpaces it will show up on all Spaces. This can also be configured in IB on the window's Attributes inspector. The behavior shouldn't differ for different users, though.