sueLan

function _isViewable(
  viewAreaMode: boolean,
  viewablePercentThreshold: number,
  top: number,
  bottom: number,
  viewportHeight: number,
  itemLength: number,
): boolean {
  if (_isEntirelyVisible(top, bottom, viewportHeight)) {
    // Entirely visible 

sueLan

this._viewableIndices = viewableIndices;
 if (this._config.minimumViewTime) {
      const handle = setTimeout(() => {
        this._timers.delete(handle);
        this._onUpdateSync(
          viewableIndices,
          onViewableItemsChanged,
          createViewToken,
        );
      }, this._config.minimumViewTime);

sueLan

// Filter out indices that have gone out of view since this call was scheduled.
    viewableIndicesToCheck = viewableIndicesToCheck.filter(ii =>
      this._viewableIndices.includes(ii),
    );

sueLan

_onUpdateSync(
    viewableIndicesToCheck,
    onViewableItemsChanged,
    createViewToken,
  ) {
    // Filter out indices that have gone out of view since this call was scheduled.
    viewableIndicesToCheck = viewableIndicesToCheck.filter(ii =>
      this._viewableIndices.includes(ii),
    );
    const prevItems = this._viewableItems;