When a MobileSafari keyboard-receing input (e.g. an input[type=text]) receives focus in Safari, the system will try to verify whether there is ample room to bring up the soft keyboard without needing to scroll. The amount of wiggle room needed for Safari to consider available scroll height "ample" seems to increase with every major iOS release (surely to work around the very real problem of focus being granted to inputs that are subsequently oscured by the keyboard).
However, for "app-ey" web sites that carefully place each text input in a relatively fixed layout, this can be a maddening arms of scooching up my content more and more with each iOS release to avoid awkward jutters every time the keyboard is shown (example video here. In practice, it seems like the input's scrollHeight can only really be about the equivalent of 15vh before you're likely to get an automated scrolling whiplash from the soft keyboard, which isn't much
