Various iOS scroll improvements (#2548)

* Don't update scrubber while post pages loading

This alleviates the scrubber bouncing around when scrolling up on iOS

* Throttle loadMore loadPrevious

Throttle loadMore and loadPrevious functions to alleviate skipping over pages and pages of posts during one scroll. This sometimes happens on iOS

js/src/forum/components/PostStream.js

@@ -146,12 +146,10 @@ export default class PostStream extends Component {
    * @param {Integer} top
    */
   onscroll(top = window.pageYOffset) {
-    if (this.stream.paused) return;
+    if (this.stream.paused || this.stream.pagesLoading) return;
 
     this.updateScrubber(top);
 
-    if (this.stream.pagesLoading) return;
-
     this.loadPostsIfNeeded(top);
 
     // Throttle calculation of our position (start/end numbers of posts in the

js/src/forum/states/PostStreamState.js

@@ -1,3 +1,4 @@
+import { throttle } from 'lodash-es';
 import anchorScroll from '../../common/utils/anchorScroll';
 
 class PostStreamState {
@@ -49,6 +50,9 @@ class PostStreamState {
      */
     this.forceUpdateScrubber = false;
 
+    this.loadNext = throttle(this._loadNext, 300);
+    this.loadPrevious = throttle(this._loadPrevious, 300);
+
     this.show(includedPosts);
   }
 
@@ -187,7 +191,7 @@ class PostStreamState {
   /**
    * Load the next page of posts.
    */
-  loadNext() {
+  _loadNext() {
     const start = this.visibleEnd;
     const end = (this.visibleEnd = this.sanitizeIndex(this.visibleEnd + this.constructor.loadCount));
 
@@ -210,7 +214,7 @@ class PostStreamState {
   /**
    * Load the previous page of posts.
    */
-  loadPrevious() {
+  _loadPrevious() {
     const end = this.visibleStart;
     const start = (this.visibleStart = this.sanitizeIndex(this.visibleStart - this.constructor.loadCount));