This article explains how to capture user input in Unreal Engine, even in UI Only mode, using a custom InputProcessor. It highlights the challenges faced when developing the Game Log System (GLS) plugin, particularly with the overlay not showing up in the main menu due to input restrictions. The article provides a solution to intercept all input events, regardless of the input mode, and demonstrates how this method ensures the overlay works on all device types.
https://dev.epicgames.com/community/learning/tutorials/KPr1/handling-input-in-ui-only-mode-a-comprehensive-guide-for-unreal-engine-developers