When I started playing B&S there was a bug that occured every now and then where the mouse pointer leaves the game window. At the time the community team said they experienced it too and they suggested switching to windowed then back to fullscreen to fix it which worked but was annoying to have to do. It happenes if the game is Windowed, Borderless Windowed or Fullscreen.
Its been 2 years and the bug still hasn't been fixed but has actually got worse. Almost every window, dialog or notification sets it off and stopping every 20 seconds in a dungeon to reset your game window is just a waste of time.