Don't use browser scroll restore in DiscussionPage (#2476)

Although native browser scroll restorations have become quite powerful, it interferes with Flarum's PostStream, so if we're on a DiscussionPage, we use manual scroll restoration.

framework/core/js/src/common/components/Page.js

@@ -29,6 +29,13 @@ export default class Page extends Component {
      * @type {Boolean}
      */
     this.scrollTopOnCreate = true;
+
+    /**
+     * Whether the browser should restore scroll state on refreshes.
+     *
+     * @type {Boolean}
+     */
+    this.useBrowserScrollRestoration = true;
   }
 
   oncreate(vnode) {
@@ -41,6 +48,10 @@ export default class Page extends Component {
     if (this.scrollTopOnCreate) {
       $(window).scrollTop(0);
     }
+
+    if ('scrollRestoration' in history) {
+      history.scrollRestoration = this.useBrowserScrollRestoration ? 'auto' : 'manual';
+    }
   }
 
   onremove() {

framework/core/js/src/forum/components/DiscussionPage.js

@@ -18,6 +18,8 @@ export default class DiscussionPage extends Page {
   oninit(vnode) {
     super.oninit(vnode);
 
+    this.useBrowserScrollRestoration = false;
+
     /**
      * The discussion that is being viewed.
      *