The function getScrollParent currently only checks style.overflowX, which is incorrect because vertical scrolling depends on overflowY.

[istarkov]

The bug is here

https://github.com/flackr/scroll-timeline/blob/master/src/scroll-timeline-base.js#L570-L585

while (node = getContainingBlock(node)) {
    const style = getComputedStyle(node);
    switch(style['overflow-x']) {
      case 'auto':
      case 'scroll':
      case 'hidden':
        // https://drafts.csswg.org/css-overflow-3/#overflow-propagation
        // The UA must apply the overflow from the root element to the viewport;
        // however, if the overflow is visible in both axis, then the overflow
        // of the first visible child body is applied instead.
        if (node == document.body &&
            getComputedStyle(document.scrollingElement).overflow == "visible")
          return  document.scrollingElement;

        return node;
    }
  }

The function getScrollParent currently only checks style.overflowX, which is incorrect because vertical scrolling depends on overflowY.
This means an element will only be considered a scroll container if it has horizontal (overflowX) scrolling.