Mouse as

Determine how your mouse works:

Trackball: you rotate etc by dragging around the screen. This is the default, and most intuitive, mode.

Joystick: you rotate etc by pressing down a mouse button off-center, and keep it pressed down. Your molecule will rotate, translate etc in the direction of your mouse button (with respect to the center of the drawing area). Since the press-and-keep-down conflicts with pop-up menus, they will be disabled.