I use MiddleMouseButton for current action, and MouseButton5 for Optics control.
These mappings stop working while I hold LeftSHIFT key (I use it as my walk key).
Any time the LeftSHIFT is held, my "look" and "perform current action" mouse buttons stop responding! The other mouse controls such as PrimaryMouse = fireWeapon are working normally.
I have a fairly standard logitech mouse, and it works normally in Arma2, and Arma3 beta until a recent Dev Update