fix(messages): default the topic message list to newest-first ordering

The default topic message view (non-continuous pagination, "Newest" /
last-50) rendered rows in the backend's stream order. The listMessages
stream isn't globally timestamp-sorted across partitions, so messages
were not shown newest-first as users expect.

`ListMessagesRequest` has no sort-order field, so ordering has to be
imposed client-side. Rather than a bespoke sort of the row list, set a
sensible default for the existing `sort` URL param so the table's own
sorted row model does the work:

- DEFAULT_SORTING is now [timestamp desc, offset desc] (offset isn't
  global across partitions, so timestamp is the primary key and offset
  is a stable tiebreak).
- getSorting() treats an empty/unset sort as "use the default", so it
  also applies for returning users whose persisted settings still hold
  the old empty sort.
- The Timestamp/Offset column headers now show the sort direction, and
  the sort is shareable/persisted via the URL.

Continuous-pagination mode disables header sorting, so the explicit
newest-first sort is kept for its "Newest" fetch (now with the same
offset tiebreak for determinism).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jvorcak

frontend/src/components/pages/topics/Tab.Messages/index.tsx

@@ -89,7 +89,7 @@ import { isServerless } from '../../../../config';
 import { useQueryStateWithCallback } from '../../../../hooks/use-query-state-with-callback';
 import { PayloadEncoding } from '../../../../protogen/redpanda/api/console/v1alpha1/common_pb';
 import { appGlobal } from '../../../../state/app-global';
-import { useTopicSettingsStore } from '../../../../stores/topic-settings-store';
+import { DEFAULT_SORTING, useTopicSettingsStore } from '../../../../stores/topic-settings-store';
 import { IsDev } from '../../../../utils/env';
 import { sanitizeString, wrapFilterFragment } from '../../../../utils/filter-helper';
 import { trimSlidingWindow } from '../../../../utils/message-table-helpers';
@@ -472,7 +472,7 @@ export const TopicMessageView: FC<TopicMessageViewProps> = (props) => {
       getDefaultValue: () => getSorting(props.topic.topicName),
     },
     'sort',
-    sortingParser.withDefault([])
+    sortingParser.withDefault(DEFAULT_SORTING)
   );
 
   // Continuous pagination toggle state
@@ -572,11 +572,16 @@ export const TopicMessageView: FC<TopicMessageViewProps> = (props) => {
       })
     : messages;
 
-  // For continuous pagination, just use the filtered messages directly
-  // We don't use placeholders as they cause page content to shift
+  // Continuous pagination disables header sorting, so the table's sorted row
+  // model can't impose newest-first there. Force it for the "Newest" fetch only
+  // (timestamp desc, then offset desc as a cross-partition tiebreak). Other
+  // origins keep the streamed order.
+  // In the normal paginated view the table sorts client-side via the default
+  // `sorting` state (DEFAULT_SORTING = timestamp desc, offset desc), so we pass
+  // the rows through untouched here.
   const filteredMessages =
     continuousPaginationEnabled && startOffset === PartitionOffsetOrigin.EndMinusResults
-      ? [...baseFilteredMessages].sort((a, b) => b.timestamp - a.timestamp)
+      ? [...baseFilteredMessages].sort((a, b) => b.timestamp - a.timestamp || b.offset - a.offset)
       : baseFilteredMessages;
 
   // Convert @computed activePreviewTags to useMemo

frontend/src/stores/topic-settings-store.ts

@@ -111,7 +111,15 @@ export type TopicSettingsStore = {
   clearAllSettings: () => void;
 };
 
-const DEFAULT_SORTING: SortingState = [];
+// Default display order for the message list: newest-first by timestamp, with
+// offset descending as a cross-partition tiebreak (offset isn't global across
+// partitions, so timestamp is the primary key). The backend's listMessages
+// stream isn't globally timestamp-sorted across partitions, so this default
+// drives the table's client-side sort to give users a sensible newest-first view.
+export const DEFAULT_SORTING: SortingState = [
+  { id: 'timestamp', desc: true },
+  { id: 'offset', desc: true },
+];
 
 const DEFAULT_SEARCH_PARAMS: TopicSearchParams = {
   offsetOrigin: -1,
@@ -214,7 +222,11 @@ export const useTopicSettingsStore = create<TopicSettingsStore>()(
 
         getSorting: (topicName: string) => {
           const topic = get().perTopicSettings.find((t) => t.topicName === topicName);
-          return topic?.searchParams.sorting ?? DEFAULT_SORTING;
+          const sorting = topic?.searchParams.sorting;
+          // Treat an empty/unset sort as "use the newest-first default" so the
+          // default applies on every fresh load, including returning users whose
+          // persisted settings still hold the old empty sort.
+          return sorting && sorting.length > 0 ? sorting : DEFAULT_SORTING;
         },
 
         setSearchParams: (topicName: string, searchParams: Partial<TopicSearchParams>) => {