That can happen if you are switching between windows while the app is initializing; executing the game and waiting until it starts should fix the issue :)
Viewing post in Neko - Desktop Pet (+18) comments
I would also like to report this error. Running Windows 10, the Unity launch screen takes up the entire screen, then it cuts to the neko in a black void. I can still click on things, but the void persists and nothing else can be made to move in front of it. Cmd key still brings up the task bar, and Task view lets me swap away from the screen without taking them (The Void or the Neko) with me.
Hope this additional data helps.