Hi,
I look at the bug report and it says this bug has been resolved, but I am having the same issue in 4.16 right now.
What I have is a MainMenu level with a widget menu and player controller for that level with input mode set to ui only. I load my main level which is from the top-down template with OpenLevel node. Inside the OnBeginPlay event on my player controller for the main level I set input mode to game only and I need to double click to move my character. Interesting thing is that if I set the input mode to game and ui it works with single clicks. Just wanted to let you know the bug is still there.