Opera 12 had mouse setup options which allowed to map mouse buttons to special operations e.g. button 8 to switch one tab left and button 9 to switch one tab right. Also the middle click paste&go anywhere in the opera window except text box was super convenient. Why where these options removed from the new design? If the idea is keeping only minimal number of options in opera://settings/ the mouse configuration could be at least in some advanced configuration place like opera://flags/ or in some extension.