Incorrect scroll height calculation clamps overflow scrolling prematurely

#### The Bug
Currently, scrollable containers (elements with overflow-y set to scroll or auto, as well as html/body tags with visible) do not always compute their full scrollable range correctly.

Relying solely on final_layout.scroll_height() can underestimate the total height of nested children, leading to early scroll clamping. Users are unable to scroll all the way down to see the bottom-most contents of a container.

#### Steps to Reproduce
1. Create a container with a fixed height and overflow-y: auto.
2. Populate it with absolute or relative layout children that extend beyond the container's height.
3. Attempt to scroll to the bottom. The scroll area is clamped early, clipping the bottom-most children.

```html

#### Expected Behavior
The scrollable height of a container should span the full bottom coordinate of all layout children plus any padding and border bounds.