IntersectionObserver offscreen bounds

Is it possible to create an Intersection Observer that only detects an intersection for vertical changes?

I tried setting a large enough horizontal rootMargin for my purposes:

{
  threshold: 1,
  rootMargin: '-57px 300% 0px 300%',
}

But the resulting boundingClientRect still seems to only intersect with the viewport:

IntersectionObserverEntry { ​
  boundingClientRect: DOMRect { 
    bottom: 441.1499938964844,
​​    height: 175.91665649414062,
​​    left: 250,
​​    right: 2167.066650390625,
​​    top: 265.23333740234375,
​​    width: 1917.066650390625,
​​    x: 250,
​​    y: 265.23333740234375
  },
​  intersectionRatio: 0.9879677284740576,
​
  intersectionRect: { 
    ​​bottom: 441.1499938964844,
    ​​height: 175.91665649414062,
    ​​left: 250,
    ​​right: 2144,
    ​​top: 265.23333740234375,
    ​width: 1894,
    ​​x: 250,
    ​​y: 265.23333740234375
​​  },
​
  isIntersecting: false,
​
  rootBounds: {
​​    bottom: 1191.4833984375,
​​    height: 1134.4833984375,
​​    left: 0,
​​    right: 8576,
​​    top: 57,
​​    width: 8576,
​​    x: 0,
​​    y: 57
  }

Why is entry.intersectionRect.right limited to 2144 when rootBounds.right is 8576?