The video is showing simulation of rotating your phone from portrait to landscape or vice-versa. Something phone and tablet users will do without even thinking about it, simply to get the vertical or horizontal real estate that allows best display of whatever they're viewing at that moment.
I see the issue you're talking about Shredder, and it happens for me (on an actual phone, Android & Chrome) in phpBB 3.2.7, too. I agree with David, it probably just needs a bug entered in the tracker
, because I don't think there is any configuration or support issue to really work through. If you don't have a Tracker account already and/or don't want one, just say so and someone else can enter it for you.
Meaning although your example was going from horizontal to vertical orientation and I see the issue there; I also see the same "wrong position" issue when going from vertical to horizontal orientation, too. It's just a matter of "which orientation were you in when the drop-down was first displayed", versus "what orientation are you in now." If it's not still the same, the "remembered" position of the drop-down menu will be wrong.