I just want to clarify that this is not an error, it is merely a warning intended for developers, FooBox is still working as intended and your site will not be impacted performance wise at all by this.
This warning is displayed as we use jQuery under the hood in FooBox for the swipe functionality on mobile devices and the way they do event bindings prevents us from passing in the “passive” option. This is on our backlog to take a look at but as it causes no harm it is marked as low priority. You can see the discussion around this still open issue on jQuery’s GitHub repo which has been open since 2016.
This warning is there to advise developers that the event they are using, in this case, touchstart, may cause the native scrolling behavior of the browser to become jittery if that event handler is called while a user is actively scrolling the page. In our scenario the touchstart event is only triggered when the lightbox is open and a user is swiping on the active item, which means they are not scrolling the page and so the warning does not actually apply.