Repro (requires touch-enabled device)

1. Set "Active Input Handling" to "Input System Package" and restart editor.
2. Add a simple MonoBehaviour script that uses GUI.Button in an OnGUI method.
3. Add it to the scene and go into play mode.
4. Try to operate the button using touch and observe there is no response.
5. Try to operate the button with the mouse and observe there is a response.

The problem here is that touch simulation for IMGUI is tied to the old input manager. Thus, when the old input backends don't run, there's no IMGUI events from touch.

For now, the only workaround is to keep "Active Input Handling" set to "Both".