Fix App Router navigation scrolling so the root element's `scroll-padding-top` defines the upper boundary of the usable viewport. During a route navigation, a destination whose top edge is hidden within that padding must not be treated as visible, while a destination that is genuinely visible below the boundary must preserve the current scroll position.

Support both pixel and percentage scroll-padding values, and apply the same visibility semantics to both the legacy element-based scroll path and the Fragment-ref scroll path. Preserve existing scroll ownership behavior: empty Fragments remain unavailable as targets, and hash navigations continue to follow their existing path.

Keep the geometry/style work efficient. Resolve the root computed style only after a real candidate yields client rects, do not perform this lookup for empty Fragments or hash navigations, and reuse the resolved padding for any second geometry check within the same navigation. Keep the value navigation-local so responsive CSS, root classes, and custom properties can change between navigations.