code: function onMouseDown(event, env, isSideEffectsDisabled) {
      mouse.down(event, env ? env.camera : null);
  
      if (!env || isSideEffectsDisabled) {
          return;
      }
  
      if (mouse.keys[1] && !mouse.keys[3]) {
          let focusedObject = focusObject(env.library, env.camera, env.selector);
  
          if (env.selector.selectFocused()) {
              events.triggerSelect(focusedObject);
          }   
      }
  }
docs: Triggers object select
  @alias module:lib3d.onMouseDown
  @param {Object} event - mouse event
  @param {Environment} env - affected environment
  @param {Boolean} isSideEffectsDisabled - disable side effects like focusing, selecting, moving objects