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/components/ui/progressiveRows.ts

@@ -18,16 +18,21 @@ export const STEP_ROWS = 40;
 // `view` names what the rows show (page, size, sort, filters). A new view starts over from the first
 // rows, so the urgent render after a click or a key stays small. New data for the same view (a refetch)
 // does not: it would drop the rows the reader is looking at.
-export function useProgressiveRows<T>(rows: T[], view: string): T[] {
+//
+// `through` is a row that must be in the DOM in THIS render: the row a page turn was made to show
+// (DataTable's `reveal`). Its page is a new view, and the caller focuses that row right after the
+// commit — at index 40 of a 50-row page it was not mounted yet, and focus fell to <body> (E31 S5).
+export function useProgressiveRows<T>(rows: T[], view: string, through = -1): T[] {
   const [seen, setSeen] = useState(view);
   const [limit, setLimit] = useState(FIRST_ROWS);
   if (seen !== view) {
     setSeen(view);
     setLimit(FIRST_ROWS);
   }
-  const more = rows.length > limit;
+  const count = Math.max(limit, through + 1);
+  const more = rows.length > count;
   useEffect(() => {
-    if (more) startTransition(() => setLimit(limit + STEP_ROWS));
-  }, [more, limit]);
-  return more ? rows.slice(0, limit) : rows;
+    if (more) startTransition(() => setLimit(count + STEP_ROWS));
+  }, [more, count]);
+  return more ? rows.slice(0, count) : rows;
 }