Fix scroll on long discussions

- Anchor scroll when inserting post placeholders
- Indicate that pages are loading at start of `loadPage`, which allows `onscroll` to not request that multiple pages be loaded at the same time

These changes are particularly applicable to firefox, where previously, dozens of posts could be skipped at a time if scroll up was held while at the top of the viewport.

js/src/forum/components/PostStream.js

@@ -149,6 +149,11 @@ export default class PostStream extends Component {
    */
   onscroll(top = window.pageYOffset) {
     if (this.stream.paused) return;
+
+    this.updateScrubber(top);
+
+    if (this.stream.pagesLoading) return;
+
     const marginTop = this.getMarginTop();
     const viewportHeight = $(window).height() - marginTop;
     const viewportTop = top + marginTop;
@@ -174,8 +179,6 @@ export default class PostStream extends Component {
     // viewport) to 100ms.
     clearTimeout(this.calculatePositionTimeout);
     this.calculatePositionTimeout = setTimeout(this.calculatePosition.bind(this, top), 100);
-
-    this.updateScrubber(top);
   }
 
   updateScrubber(top = window.pageYOffset) {

js/src/forum/states/PostStreamState.js

@@ -238,23 +238,26 @@ class PostStreamState {
    * @param {Boolean} backwards
    */
   loadPage(start, end, backwards = false) {
-    m.redraw();
+    this.pagesLoading++;
+
+    const redraw = () => {
+      if (start < this.visibleStart || end > this.visibleEnd) return;
+
+      const anchorIndex = backwards ? this.visibleEnd - 1 : this.visibleStart;
+      anchorScroll(`.PostStream-item[data-index="${anchorIndex}"]`, m.redraw.sync);
+    };
+    redraw();
 
     this.loadPageTimeouts[start] = setTimeout(
       () => {
         this.loadRange(start, end).then(() => {
-          if (start >= this.visibleStart && end <= this.visibleEnd) {
-            const anchorIndex = backwards ? this.visibleEnd - 1 : this.visibleStart;
-            anchorScroll(`.PostStream-item[data-index="${anchorIndex}"]`, () => m.redraw.sync());
-          }
+          redraw();
           this.pagesLoading--;
         });
         this.loadPageTimeouts[start] = null;
       },
-      this.pagesLoading ? 1000 : 0
+      this.pagesLoading - 1 ? 1000 : 0
     );
-
-    this.pagesLoading++;
   }
 
   /**