fix(ui): a read-ahead waits out a background refetch, and a revealed row is mounted before it is focused

NEXT_PAGE joins ANY fetch in flight: a read-ahead during an invalidation
or a poll joined that refetch and appended no page, and nothing asked
again until the reader moved the range (the channel page during a
backfill). The read-ahead now waits while the list fetches anything
(mayReadAhead, re-run on isFetching); a Load older click cancels a poll
instead (nextPageOnClick).

A page turn to a revealed row restarted the progressive mount at 30
rows; Channels focuses that row in the next commit, and at index 40 of
a 50-row page it was not there, so focus fell to <body>. The mount now
reaches through the revealed row.

frontend/src/lib/nextPage.dom.test.ts

@@ -5,9 +5,22 @@
 import { InfiniteQueryObserver } from "@tanstack/react-query";
 import { describe, expect, it } from "vitest";
 
-import { NEXT_PAGE } from "./nextPage";
+import { mayReadAhead, NEXT_PAGE, nextPageOnClick } from "./nextPage";
 import { createQueryClient } from "./queryClient";
 
+function probe(sent: (string | null)[], delay = 5) {
+  return new InfiniteQueryObserver(createQueryClient(), {
+    queryKey: ["probe"],
+    initialPageParam: null as string | null,
+    queryFn: async ({ pageParam }) => {
+      sent.push(pageParam);
+      await new Promise((r) => setTimeout(r, delay));
+      return { next: `c${sent.length}` };
+    },
+    getNextPageParam: (last: { next: string }) => last.next,
+  });
+}
+
 describe("NEXT_PAGE", () => {
   it("sends each page once when the read-ahead calls twice in a row", async () => {
     const sent: (string | null)[] = [];
@@ -32,4 +45,46 @@ describe("NEXT_PAGE", () => {
     expect(observer.getCurrentResult().data?.pages).toHaveLength(2);
     unsubscribe();
   });
+
+  // E31 S5: the join takes ANY fetch in flight. During a background refetch (an invalidation, a
+  // poll) it reloads the pages already there and appends none — why a read-ahead waits it out.
+  it("joins a background refetch and appends no page", async () => {
+    const sent: (string | null)[] = [];
+    const observer = probe(sent);
+    const unsubscribe = observer.subscribe(() => {});
+    await observer.refetch();
+
+    void observer.refetch();
+    await observer.fetchNextPage(NEXT_PAGE);
+
+    expect(observer.getCurrentResult().data?.pages).toHaveLength(1);
+    unsubscribe();
+  });
+});
+
+describe("a read-ahead", () => {
+  it("waits while the list fetches anything", () => {
+    expect(mayReadAhead(true, false)).toBe(true);
+    expect(mayReadAhead(true, true)).toBe(false);
+    expect(mayReadAhead(false, false)).toBe(false);
+  });
+});
+
+describe("a next page asked for by a click", () => {
+  it("loads the page even while a background refetch runs", async () => {
+    const sent: (string | null)[] = [];
+    const observer = probe(sent);
+    const unsubscribe = observer.subscribe(() => {});
+    await observer.refetch();
+
+    void observer.refetch();
+    await observer.fetchNextPage(nextPageOnClick(false));
+
+    expect(observer.getCurrentResult().data?.pages).toHaveLength(2);
+    unsubscribe();
+  });
+
+  it("joins a next page already loading", () => {
+    expect(nextPageOnClick(true)).toBe(NEXT_PAGE);
+  });
 });