Phantom Alt-Tab Error

I am running the game in full screen mode. Usually early in the first game after logging in (5-8 minutes), the game disappears from the screen as if I had alt-tabbed. Seems like it happens in the middle of team fights when I flash or ignite. The game disappears from the screen, but the sound conveniently streams my death as I am trying to get game video active on the screen again. I run two screens (should this mean anything). And the issue seems to have begun after the latest patch. It only seems to happen in the first game after each login. But this is very annoying and, I am sure, confusing to my teammates.

UPDATE (9/23/19): I have been able to reproduce the problem and identify the cause. I have some hot keys mapped to my mouse. In the first game after logging in, the phantom alt-tab error occurs when I press Mouse Button 4. After re-entering the game, the error does not recur in that or later games in the same login session. Outside of the game, Mouse Button 4 is mapped to "previous" action.

Hopefully this is helpful. Knowing the cause of the error means it is now more of an annoyance than debilitating, but it really should not be happening at all.