Try to improve the user experience when a scrollResponseBoundary of a reasonable size is set.

In _updateScroll, adjust the current scroll based on the instance's scrollResponseBoundary setting. This means the scroll doesn't jump to the gesture position immediately, instead, the scroll position lags behind by the scroll response boundary value.    Described as 'glitchy article scrolling': https://redmine.labs.ft.com/issues/34972