Expose virt list scroll variables to window (mattermost#2569)

components/post_view/post_list.jsx

@@ -21,6 +21,9 @@ const POSTS_PER_PAGE = Constants.POST_CHUNK_SIZE / 2;
 const MAX_NUMBER_OF_AUTO_RETRIES = 3;
 
 const MAX_EXTRA_PAGES_LOADED = 10;
+const OVERSCAN_COUNT_BACKWARD = window.OVERSCAN_COUNT_BACKWARD || 50; // Exposing the value for PM to test will be removed soon
+const OVERSCAN_COUNT_FORWARD = window.OVERSCAN_COUNT_FORWARD || 100; // Exposing the value for PM to test will be removed soon
+const HEIGHT_TRIGGER_FOR_MORE_POSTS = window.HEIGHT_TRIGGER_FOR_MORE_POSTS || 1000; // Exposing the value for PM to test will be removed soon
 
 export default class PostList extends React.PureComponent {
  static propTypes = {
@@ -299,7 +302,7 @@ export default class PostList extends React.PureComponent {
  onScroll = ({scrollDirection, scrollOffset, scrollUpdateWasRequested}) => {
  const isNotLoadingPosts = !this.state.loadingFirstSetOfPosts && !this.loadingMorePosts;
  const didUserScrollBackwards = scrollDirection === 'backward' && !scrollUpdateWasRequested;
- const isOffsetWithInRange = scrollOffset < 1000;
+ const isOffsetWithInRange = scrollOffset < HEIGHT_TRIGGER_FOR_MORE_POSTS;
  if (isNotLoadingPosts && didUserScrollBackwards && isOffsetWithInRange && !this.state.atEnd) {
  this.loadingMorePosts = true;
  this.loadMorePosts();
@@ -473,7 +476,8 @@ export default class PostList extends React.PureComponent {
  itemCount={this.state.postListIds.length}
  itemData={this.state.postListIds}
  itemKey={this.itemKey}
- overscanCount={100}
+ overscanCountForward={OVERSCAN_COUNT_FORWARD}
+ overscanCountBackward={OVERSCAN_COUNT_BACKWARD}
  onScroll={this.onScroll}
  onItemsRendered={this.onItemsRendered}
  initScrollToIndex={this.initScrollToIndex}