Why do my UI clicks behave erratically with additive scenes?

Each scene shipped with its own EventSystem. After Additive load, you have two; one wins arbitrarily. Keep one global EventSystem with DontDestroyOnLoad and strip from additive scenes.

How to enforce singleton?

In a bootstrap script: if (FindObjectsOfType<EventSystem>().Length > 1) destroy duplicates. Or simply remove from additive scene prefabs.

Unity logs 'There are 2 event systems in the scene. Please ensure there is only one event system in the scene.' Catch this in CI.

Fix: Multiple Unity EventSystems Conflict

Quick answer: One EventSystem global, marked DontDestroyOnLoad. Remove EventSystem GameObjects from additive scenes.

Bootstrap scene + Additive UI scene. Both have an EventSystem. UI works in editor, breaks after additive load.

The Fix

BootstrapScene:
  GameObject EventSystem (with EventSystem + InputSystemUIInputModule)
    DontDestroyOnLoad(this) on Awake

AdditiveScenes:
  Remove EventSystem GameObjects entirely

// Optional bootstrap guard
void Awake() {
    var all = FindObjectsOfType<EventSystem>();
    if (all.Length > 1) {
        foreach (var es in all)
            if (es != this.GetComponent<EventSystem>())
                Destroy(es.gameObject);
    }
}

Single global event system means consistent input routing. DontDestroyOnLoad survives scene transitions.

Verifying

Console clean. UI clicks land consistently across additive scene swaps.

“One EventSystem. UI consistent.”

Related Issues

For UI Graphic Raycaster, see Raycaster. For TMP InputField caret, see TMP caret.

One EventSystem. UI consistent.