fix: mobile discussion total post count not updating on reply (#4684)

The scrubber's "viewing X of Y" label was built once as a vnode tree
and rendered in two places (the dropdown toggle and the Scrubber-info
strong). Reusing a single vnode instance in two positions breaks
Mithril's diffing, leaving one copy stale on redraw.

The current post index is force-written to the DOM via jQuery on
scroll, so it always updated; the total count is only rendered through
Mithril's view, so the stale copy (the dropdown toggle shown in the
mobile header) never refreshed after posting a reply.

Build the label as a function that returns a fresh vnode tree per
usage so both render sites diff correctly.

Backport fix #4678

Co-authored-by: IanM <16573496+imorland@users.noreply.github.com>

Author: huoxin233

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

@@ -26,12 +26,16 @@ export default class PostStreamScrubber extends Component {
   view() {
     const count = this.stream.count();
 
-    // Index is left blank for performance reasons, it is filled in in updateScubberValues
-    const viewing = app.translator.trans('core.forum.post_scrubber.viewing_text', {
-      count,
-      index: <span className="Scrubber-index"></span>,
-      formattedCount: <span className="Scrubber-count">{formatNumber(count)}</span>,
-    });
+    // Index is left blank for performance reasons, it is filled in in updateScubberValues.
+    // Built as a function so each usage renders its own vnode tree — reusing a single vnode
+    // instance in two places breaks Mithril's diffing and leaves one copy stale (e.g. the
+    // total count in the mobile header not updating after posting a reply).
+    const viewing = () =>
+      app.translator.trans('core.forum.post_scrubber.viewing_text', {
+        count,
+        index: <span className="Scrubber-index"></span>,
+        formattedCount: <span className="Scrubber-count">{formatNumber(count)}</span>,
+      });
 
     const unreadCount = this.stream.discussion.unreadCount();
     const unreadPercent = count ? Math.min(count - this.stream.index, unreadCount) / count : 0;
@@ -58,7 +62,7 @@ export default class PostStreamScrubber extends Component {
     return (
       <div className={classNames.join(' ')}>
         <button className="Button Dropdown-toggle" data-toggle="dropdown">
-          {viewing} {icon('fas fa-sort')}
+          {viewing()} {icon('fas fa-sort')}
         </button>
 
         <div className="Dropdown-menu dropdown-menu">
@@ -72,7 +76,7 @@ export default class PostStreamScrubber extends Component {
               <div className="Scrubber-handle">
                 <div className="Scrubber-bar" />
                 <div className="Scrubber-info">
-                  <strong>{viewing}</strong>
+                  <strong>{viewing()}</strong>
                   <span className="Scrubber-description"></span>
                 </div>
               </div>