Hey Jonathan,
Looks like the problem is that the game viewport is only setting keyboard focus when the window is focused (the keyboard == controller 1). So even after left-clicking, if you alt+tab away from your PIE viewport and alt+tab back to it, once again only controller 1 will be working. This is certainly something we’ll have to check out further and fix in the future.
In the meantime, there’s thankfully a much more preferable workaround to forcing a click. The blueprint library function from UWidgetBlueprintLibrary called SetFocusToGameViewport. Since it’s provided by a blueprint library, it can be accessed by any blueprint. If you’re using C++, it’s equivalent to calling FSlateApplication::Get().SetAllUserFocusToGameViewport().
The only caveat here is that the game viewport needs to have been registered already, which hasn’t happened when the level blueprint’s BeginPlay event fires (that’s also something we’ll try to fix). So, the quick and dirty way to fix this is to register a one second timer that triggers a call to SetFocusToGameViewport. Of course, if you have a better spot to put it after BeginPlay fires, by all means :).
-DanH