ant-design/components/affix/utils.ts

export type BindElement = HTMLElement | Window | null | undefined;

export function getTargetRect(target: BindElement): DOMRect {
  return target !== window
    ? (target as HTMLElement).getBoundingClientRect()
    : ({ top: 0, bottom: window.innerHeight } as DOMRect);
}

export function getFixedTop(placeholderRect: DOMRect, targetRect: DOMRect, offsetTop?: number) {
  if (offsetTop !== undefined && targetRect.top > placeholderRect.top - offsetTop) {
    return offsetTop + targetRect.top;
  }
  return undefined;
}

export function getFixedBottom(
  placeholderRect: DOMRect,
  targetRect: DOMRect,
  offsetBottom?: number,
) {
  if (offsetBottom !== undefined && targetRect.bottom < placeholderRect.bottom + offsetBottom) {
    const targetBottomOffset = window.innerHeight - targetRect.bottom;
    return offsetBottom + targetBottomOffset;
  }
  return undefined;
}