block12.js

for (let idx = first; idx <= last; idx++) {
      const metrics = getFrameMetrics(idx);
      if (!metrics) {
        continue;
      }
      // The top of current item cell, relative to the screen 
      const top = metrics.offset - scrollOffset;
      // The bottom of current item cell 
      const bottom = top + metrics.length;
      if (top < viewportHeight && bottom > 0) {
        firstVisible = idx;
        if (
          _isViewable(
            viewAreaMode,
            viewablePercentThreshold,
            top,
            bottom,
            viewportHeight,
            metrics.length,
          )
        ) {
          viewableIndices.push(idx);
        }
      } else if (firstVisible >= 0) {
        break;
      }
    }
    return viewableIndices;
  }