Wow, 4 years years later and this is still a problem. I’m able to reproduce this 100% of the time. I am in Fullscreen mode. Placing my cursor on the edge of my in-game screen (adjacent to my second monitor), left clicking to look around (in a way that would move my cursor towards the direction of my second monitor), and releasing my left click causes the cursor to appear on my second monitor. Left clicking or right clicking while the cursor is active on my second monitor will minimize the game. As others have said, this is very frustrating. Fullscreen should confine the cursor to the game!
Running a Windows 10 system.