fix(ui): keep Lens investigation preview rows in place while matches reload

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ui/litellm-dashboard/src/components/lens/setup/InvestigationSetup.integration.test.tsx

@@ -430,6 +430,30 @@ it("fetches one preview for two keystrokes inside the debounce window", async ()
   }
 });
 
+it("shows skeleton rows on first load and keeps the previous rows while a new search loads", async () => {
+  const pending = new Map<string, (page: unknown) => void>();
+  proxy.post.mockImplementation(
+    (_path, options) =>
+      new Promise((resolve) => {
+        pending.set((options?.body as { selection: Settings }).selection.q, resolve);
+      }),
+  );
+  const user = userEvent.setup();
+  renderWithProviders(<InvestigationSetup mode="new" onClose={vi.fn()} onSave={vi.fn()} />);
+  const preview = within(screen.getByRole("region", { name: "Matching activity" }));
+  expect((await preview.findAllByTestId("runs-placeholder")).length).toBeGreaterThan(0);
+  act(() => pending.get("")?.({ eligible: 1, selected: 1, executions: [run("Old run", 1)] }));
+  expect(await preview.findByText("Old run")).toBeVisible();
+  expect(preview.queryByTestId("runs-placeholder")).not.toBeInTheDocument();
+  await user.type(screen.getByRole("combobox", { name: "Search runs" }), "new");
+  await waitFor(() => expect(pending.has("new")).toBe(true));
+  expect(preview.getByText("Old run")).toBeVisible();
+  expect(preview.queryByTestId("runs-placeholder")).not.toBeInTheDocument();
+  act(() => pending.get("new")?.({ eligible: 1, selected: 1, executions: [run("New run", 2)] }));
+  expect(await preview.findByText("New run")).toBeVisible();
+  expect(preview.queryByText("Old run")).not.toBeInTheDocument();
+});
+
 it.each(["empty", "error"])("blocks a new investigation when its preview is %s", async (state) => {
   if (state === "error") proxy.post.mockRejectedValue(new Error("Storage unavailable"));
   else proxy.post.mockResolvedValue({ eligible: 0, selected: 0, executions: [] });

ui/litellm-dashboard/src/components/lens/setup/MatchingActivityPreview.tsx

@@ -60,6 +60,7 @@ export function MatchingActivityPreview({
   ...props
 }: MatchingActivityPreviewProps) {
   const runs = page.executions.flatMap((run) => (run.summary ? [run.summary] : []));
+  const shown = status.ready || status.stale;
   return (
     <section
       aria-label="Matching activity"
@@ -97,21 +98,22 @@ export function MatchingActivityPreview({
           No matches. Try removing a filter from the search. Recent trace updates need two minutes to settle.
         </p>
       )}
-      {status.ready && runs.length > 0 && (
+      {(status.loading || (shown && !status.error && runs.length > 0)) && (
         <div className="max-h-[60dvh] min-h-0 overflow-auto">
           <AgentTracesTable
             traces={runs}
-            isLoading={false}
+            isLoading={status.loading}
             error={null}
             hasMore={page.hasMore}
             isFetching={page.loadingMore}
+            isPlaceholder={status.stale}
             onLoadMore={page.loadMore}
             onSetUpTracing={() => {}}
             picks={selection ? runPicks(selection) : undefined}
           />
         </div>
       )}
-      {status.ready && <PreviewFooter page={page} selection={selection} />}
+      {shown && <PreviewFooter page={page} selection={selection} />}
     </section>
   );
 }

ui/litellm-dashboard/src/components/lens/setup/useMatchingActivity.ts

@@ -29,6 +29,10 @@ export interface PreviewStatus {
   readonly title: string;
   readonly windowLabel: string;
   readonly ready: boolean;
+  /** No results yet for any search: the table shows skeleton rows. */
+  readonly loading: boolean;
+  /** The rows belong to the previous search while this one loads. */
+  readonly stale: boolean;
   readonly error: Error | null;
   readonly refresh: () => void;
 }
@@ -144,6 +148,8 @@ export function useMatchingActivity(): MatchingActivity {
   }, [empty, valid, asOf, refresh]);
   const pending = settling || preview.isLoading || preview.isPlaceholderData;
   const ready = !pending && valid;
+  const loading = valid && pending && !firstPage;
+  const stale = valid && pending && !!firstPage;
   const setExecutionIds = (next: readonly string[]) =>
     setValue("selection.execution_ids", [...next], { shouldValidate: true });
   const picked = manualSelection ? manualPicks(selection, setExecutionIds) : null;
@@ -156,6 +162,8 @@ export function useMatchingActivity(): MatchingActivity {
         title: previewTitle({ pending, validWindow: windowValid, valid }, firstPage),
         windowLabel: windowLabel(selection),
         ready,
+        loading,
+        stale,
         error: preview.error,
         refresh,
       },