I tried creating a widget with a button, and when I entered fullscreen I was able to click the button as expected. Could you provide the setup/repro steps that led to the behavior you’re seeing?
I had a bit of time to look at this today. In a nutshell, it’s the Ubuntu launcher interfering somehow with the UMG widget click. Funny thing is, I seem able to interact with the world fine along that edge, but the UMG widgets are not receiving any events.
I got my colleague to test it on his Ubuntu machine, sure enough, he experienced the same problem. I wonder if, in your test, the launcher was set to another monitor or it was perhaps set to auto-hide when the user isn’t interacting with it?