Scrolling fixes (#3200)

* scrolling fixes:
- jumps on free scroll when somebody posts a new message (and old one disappears at top)
- jumps on closing RHS (no compensation of resize)
- scroll blinking on opening/closing RHS (delay before resize compensation)

* fix scroll slipping up on multiple open/close RHS in case when (this.refs.postlist.offsetHeight % Constants.SCROLL_PAGE_FRACTION === 0)

webapp/components/post_view/components/post_list.jsx

@@ -68,7 +68,7 @@ export default class PostList extends React.Component {
         const childNodes = this.refs.postlistcontent.childNodes;
         for (let i = 0; i < childNodes.length; i++) {
             // If the node is 1/3 down the page
-            if (childNodes[i].offsetTop > (this.refs.postlist.scrollTop + (this.refs.postlist.offsetHeight / Constants.SCROLL_PAGE_FRACTION))) {
+            if (childNodes[i].offsetTop >= (this.refs.postlist.scrollTop + (this.refs.postlist.offsetHeight / Constants.SCROLL_PAGE_FRACTION))) {
                 this.jumpToPostNode = childNodes[i];
                 break;
             }
@@ -367,9 +367,8 @@ export default class PostList extends React.Component {
             }
         } else if (this.refs.postlist.scrollHeight !== this.prevScrollHeight) {
             window.requestAnimationFrame(() => {
-                // Only need to jump if we added posts to the top.
-                if (this.jumpToPostNode && (this.jumpToPostNode.offsetTop !== this.prevOffsetTop)) {
-                    this.refs.postlist.scrollTop += (this.refs.postlist.scrollHeight - this.prevScrollHeight);
+                if (this.jumpToPostNode) {
+                    this.refs.postlist.scrollTop += (this.jumpToPostNode.offsetTop - this.prevOffsetTop);
                 }
             });
         }