Function getMenuPlacement

packages/react-select/src/components/Menu.tsx:59–219

({
  maxHeight: preferredMaxHeight,
  menuEl,
  minHeight,
  placement: preferredPlacement,
  shouldScroll,
  isFixedPosition,
  controlHeight,
}: PlacementArgs)

}

export function getMenuPlacement({
  maxHeight: preferredMaxHeight,
  menuEl,
  minHeight,
  placement: preferredPlacement,
  shouldScroll,
  isFixedPosition,
  controlHeight,
}: PlacementArgs): CalculatedMenuPlacementAndHeight {
  const scrollParent = getScrollParent(menuEl!);
  const defaultState: CalculatedMenuPlacementAndHeight = {
    placement: 'bottom',
    maxHeight: preferredMaxHeight,
  };

  // something went wrong, return default state
  if (!menuEl || !menuEl.offsetParent) return defaultState;

  // we can't trust `scrollParent.scrollHeight` --> it may increase when
  // the menu is rendered
  const { height: scrollHeight } = scrollParent.getBoundingClientRect();
  const {
    bottom: menuBottom,
    height: menuHeight,
    top: menuTop,
  } = menuEl.getBoundingClientRect();

  const { top: containerTop } = menuEl.offsetParent.getBoundingClientRect();
  const viewHeight = isFixedPosition
    ? window.innerHeight
    : normalizedHeight(scrollParent);
  const scrollTop = getScrollTop(scrollParent);

  const marginBottom = parseInt(getComputedStyle(menuEl).marginBottom, 10);
  const marginTop = parseInt(getComputedStyle(menuEl).marginTop, 10);
  const viewSpaceAbove = containerTop - marginTop;
  const viewSpaceBelow = viewHeight - menuTop;
  const scrollSpaceAbove = viewSpaceAbove + scrollTop;
  const scrollSpaceBelow = scrollHeight - scrollTop - menuTop;

  const scrollDown = menuBottom - viewHeight + scrollTop + marginBottom;
  const scrollUp = scrollTop + menuTop - marginTop;
  const scrollDuration = 160;

  switch (preferredPlacement) {
    case 'auto':
    case 'bottom':
      // 1: the menu will fit, do nothing
      if (viewSpaceBelow >= menuHeight) {
        return { placement: 'bottom', maxHeight: preferredMaxHeight };
      }

      // 2: the menu will fit, if scrolled
      if (scrollSpaceBelow >= menuHeight && !isFixedPosition) {
        if (shouldScroll) {
          animatedScrollTo(scrollParent, scrollDown, scrollDuration);
        }