Hey Eric,
The button currently does nothing. I was just getting started.
Did Slate, Hello tutorial.
Added a button.
Play In Editor with current Viewport. Clicked and held down on the button and → game pauses while mouse is down.
It’s a CompoundWidget in the game, created and attached through the hud, not a tool window in the editor.
This is on the 4.3 preview, maybe this has already been resolved?