I noticed this strange behavior which breaks my UI… every widget will fire On Mouse Leave events even when the Input Mode is set to game only.
I have a few widgets that play a brief animation (like fade in/out) when the mouse cursors enters or leaves but even when I set the input mode to game only and hide the cursor, the mouse leave events will still fire as I move the mouse around (the mouse enter events don’t fire, which is the correct behavior in my opinion).
Also… buttons, combo-boxes (and possibly more things) can still be clicked even when the input mode is set to game only and cursor is hidden.
I was able to reproduce this issue on our end. After some digging I found that this is a known issue (UE-13719) and that it has been submitted to the developers for further consideration. I will be sure to bump the community interest for this issue. I was also able to come up with a workaround that may be viable depending on your needs. You could set the “Visibility” of the widgets to “Self Hit Test Invisible”. This will allow them to stay on screen but they will no longer interact with the mouse. This could also be a solution to the second issue that you mentioned at the end of your post. I hope that this helps.