title: react-window and infinite loader scrolling issue
body: <p>I have given an arbitrary number in itemCount because my list length is unknown.
  When I scroll down, infiniteloader loads the data normally for the first time only. But then it loads after my window view is completely blank.</p>
  
  <p>Let's say my list of array contains 10 items on every fetch.
  How can I load data as soon as I scroll down and show content in time?</p>
  
  <pre><code>const CARD_SIZE = 265;
  
  class CardList extends PureComponent {
  
      getItemData = memoize((itemsPerRow, newItems) =&gt; ({
          itemsPerRow,
          newItems
      }))
  
  
          Row = ({data, index, style }) =&gt; {
          const { itemsPerRow, newItems } = data;
  
          const items = [];
          const fromIndex = index * itemsPerRow;
          const toIndex = Math.min(fromIndex + itemsPerRow, newItems.length);
  
          for (let i = fromIndex; i &lt; toIndex; i++) {
              items.push(
                  &lt;Card 
                  key={newItems[i]}
                  style={style}
                  token={this.props.token}
                  disableButton={this.props.disableButton} 
                  image={newItems[i]} /&gt;
                  );
          }
              return (
                  &lt;div style={style}&gt;
                  {items}
                  &lt;/div&gt;
              );
          }
  
      render() {
  
          const { newItems, loadMore } = this.props;
  
          return (
              &lt;div style={{marginTop: "10px", height: "100vh" }}&gt;
  
              &lt;AutoSizer&gt;
              {
                  ({ height, width }) =&gt; {
                      const itemsPerRow = Math.floor(width / CARD_SIZE) || 1;
                      // const rowCount = Math.ceil(newItems.length / itemsPerRow);
                      const itemData = this.getItemData(itemsPerRow, newItems);
  
                      return (
                &lt;InfiniteLoader
                isItemLoaded={index =&gt; index &lt; newItems.length - 1}
                itemCount={5000}
                loadMoreItems={loadMore}
                 &gt;
                {({ onItemsRendered, ref }) =&gt; (
  
                          &lt;List
                          height={height}
                          itemCount={5000}
                          itemData={itemData}
                          itemSize={CARD_SIZE}
                          width={width}
                          overscanCount={1}
                          onItemsRendered={onItemsRendered}
                          ref={ref}
                          &gt;
                          { this.Row }
                          &lt;/List&gt;
  
                          )}
              &lt;/InfiniteLoader&gt;
  
                          )
                  }
              }
              &lt;/AutoSizer&gt; 
              &lt;/div&gt;
              );
      }
  }
  </code></pre>