First of all, can I just say that’s an adorable login page?!!!
Anyway, to the matter at hand. Just to clarify: when you let go, the screen kind of snaps back to the content, right?
I visited the hotbed login page on my phone and couldn’t get it to scroll to the side like this. The only way I could scroll to the side is if I zoomed in on the page a bit first (like with a reverse-pinch motion). Then I could pull the page over like this. But on initial load I could not. If I zoomed in at all, even just an imperceptible amount, then I could reproduce. And trying this on other sites (facebook, reddit) exhibits similar behavior.
In general, behavior on mobile is such that as long as content does not overflow to the left or right, the page will not be scrollable horizontally. However if you have something that is overflowing to the side, then the page will be scrollable horizontally so the user can see the content. So when someone zooms in, it makes the content overflow to the right and the left, so you need to be able to scroll right and left (to see the content).
It used to be possible to disable zooming on mobile devices but for various reasons this is no longer recommended and has been disabled by Apple and will likely be disabled on Android devices as well soon. (As an aside, I saw a post once from a visually impaired person who said sites that on sites that disabled zooming, they had to take a screenshot of the page and then zoom in on the screenshot to be able to read the text! )
If you want to ensure that overflowing content is cut off on mobile and therefore not horizontally scrollable (again, if a user zooms in they’ll still be able to scroll), I bet we could cook up some custom CSS to do that. This usually involves
overflow: hidden on parent elements.