Check if we should hydrate messages below the current view.
Maximum scroll offset of the viewport, when known. When the view is scrolled to this edge but history is still archived below, hydration must run regardless of the spacer-distance heuristic: the user cannot scroll any further, and estimated spacer heights can drift from the real DOM layout enough to leave the distance check just short of its threshold, stranding the tail. Mirrors how scrolling to the top (offset 0) always hydrates above.
