I'm amazed that this is still an issue...
Just to add some more information, it also happens on Windows 10 (even though I doubt this has anything to do with OS version).

It (rightclicking to zoom in a weapon after having checked inventory, only to have arma minimize and me rightclicking for instance Teamspeak on a secondary monitor) usually happens a few times per gamenight, and when you need to keep tabs on several layers of radio coms, having to deal with "randomly" minimizing arma windows is not something you want to stack ontop of it all.