Use IMGUI alongside the Input System package

The Input System package doesn't support Immediate Mode GUI (IMGUI) methods at runtime. However, if you need to use IMGUI for your UI, you can use legacy Input Manager input for IMGUI, and the Input System package for your in-game input.

When the Unity Editor's Active Input Handling setting is set to Input System Package (which is the default, when using the Input System package), the OnGUI methods in your player code don't receive any input events.

To restore functionality to runtime OnGUI methods, you can change the Active Input Handling setting to Both. Doing this means that Unity processes the input twice, which might introduce a small performance impact.

This only affects runtime (Play mode) OnGUI methods. Editor GUI code is unaffected and continues to receive input events.