Encapsulate viewingEnd() in state

...instead of calculating this derived value outside the state class.

js/src/forum/components/PostStream.js

@@ -45,7 +45,7 @@ export default class PostStream extends Component {
     let lastTime;
 
     this.state.visibleEnd = this.state.sanitizeIndex(this.state.visibleEnd);
-    this.state.viewingEnd = this.state.visibleEnd === this.state.count();
+    const viewingEnd = this.state.viewingEnd();
 
     const posts = this.state.posts();
     const postIds = this.state.discussion.postIds();
@@ -94,7 +94,7 @@ export default class PostStream extends Component {
       );
     });
 
-    if (!this.state.viewingEnd && posts[this.state.visibleEnd - this.state.visibleStart - 1]) {
+    if (!viewingEnd && posts[this.state.visibleEnd - this.state.visibleStart - 1]) {
       items.push(
         <div className="PostStream-loadMore" key="loadMore">
           <Button className="Button" onclick={this.state.loadNext.bind(this)}>
@@ -106,7 +106,7 @@ export default class PostStream extends Component {
 
     // If we're viewing the end of the discussion, the user can reply, and
     // is not already doing so, then show a 'write a reply' placeholder.
-    if (this.state.viewingEnd && (!app.session.user || this.state.discussion.canReply())) {
+    if (viewingEnd && (!app.session.user || this.state.discussion.canReply())) {
       items.push(
         <div className="PostStream-item" key="reply">
           {ReplyPlaceholder.component({ discussion: this.state.discussion })}

js/src/forum/states/PostStreamState.js

@@ -148,6 +148,15 @@ class PostStreamState {
     return this.discussion.postIds().length;
   }
 
+  /**
+   * Are we currently viewing the end of the discussion?
+   *
+   * @return {boolean}
+   */
+  viewingEnd() {
+    return this.visibleEnd === this.count();
+  }
+
   /**
    * Make sure that the given index is not outside of the possible range of
    * indexes in the discussion.
@@ -166,7 +175,7 @@ class PostStreamState {
    * @public
    */
   update() {
-    if (!this.viewingEnd) return m.deferred().resolve().promise;
+    if (!this.viewingEnd()) return m.deferred().resolve().promise;
 
     this.visibleEnd = this.count();