When you’re in fullscreen and you have a second monitor and you go to the very edge of the monitor (with your mouse pointer) you’re playing Overwatch on and leftclick, the game alt + tabs out. This is a big issue when you’re ingame and you flick very fast in the direction of the second monitor and want to shoot but instead the Overwatch screen just switches to Discord or something for example. This happened to me and my friend A LOT while playing Overwatch for months now. Thanks for your help!
I recommend you set your window to borderless windowed instead of fullscreen.
This should fix the issue.