perf(ui): a long table page paints its first rows at once and the rest in steps

The channel manager's All page (~400 rows) blocked one frame for 0.7-0.9 s on prod to mount,
and every sort and search key re-rendered every row (300-400 ms). No single cell is to blame,
measured column by column. DataTable now mounts the first 30 rows at once and 40 more per
background transition; a new page, size, sort, filter or caller-side search (narrowedBy)
starts over from the first rows. A row is its own component, so the compiler keeps the rows a
step already holds.

frontend/src/components/ChannelDiscovery.tsx

@@ -242,6 +242,7 @@ export default function ChannelDiscovery({
             onSortChange={setSort}
             emptyText={t("channels.discovery.empty")}
             keyboardNav
+            narrowedBy={q}
           />
         ) : (
           <ChannelLayoutGrid

frontend/src/components/Channels.tsx

@@ -754,6 +754,7 @@ export default function Channels() {
                 keyboardNav
                 reveal={follow}
                 surfacedFilters={SURFACED_FILTERS}
+                narrowedBy={q}
               />
             ) : (
               <>

frontend/src/components/ui/DataTable.progressive.dom.test.tsx

@@ -0,0 +1,103 @@
+// @vitest-environment jsdom
+//
+// E31 S4d: the channel manager's "All" page (~400 rows) blocked one frame for 0.7–0.9 s on prod to
+// mount, and every key in its search re-rendered every row. A long page now mounts its first rows at
+// once and the rest in small background steps (progressiveRows), and a step leaves the rows it holds
+// alone (a row is its own component, whose cells the React Compiler caches).
+//
+// These tests drive React without act(): act flushes the background steps along with the first render,
+// and the first render on its own is the property under test.
+import { describe, it, expect, afterEach, beforeAll, afterAll, vi } from "vitest";
+import { flushSync } from "react-dom";
+import { createRoot, type Root } from "react-dom/client";
+import { waitFor } from "@testing-library/react";
+import DataTable, { type Column } from "./DataTable";
+import { FIRST_ROWS } from "./progressiveRows";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
+});
+
+const g = globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean };
+let wasAct: boolean | undefined;
+beforeAll(() => {
+  wasAct = g.IS_REACT_ACT_ENVIRONMENT;
+  g.IS_REACT_ACT_ENVIRONMENT = false;
+});
+afterAll(() => {
+  g.IS_REACT_ACT_ENVIRONMENT = wasAct;
+});
+
+type Row = { id: string; name: string };
+const many = (n: number, prefix = "r"): Row[] =>
+  Array.from({ length: n }, (_, i) => ({ id: `${prefix}${i}`, name: `${prefix} ${i}` }));
+
+let root: Root | null = null;
+let host: HTMLElement | null = null;
+afterEach(() => {
+  root?.unmount();
+  host?.remove();
+  root = null;
+  host = null;
+});
+
+const bodyRows = () => host!.querySelectorAll("tbody tr").length;
+
+// Mounts synchronously, so what the DOM holds right after is the first render alone.
+function mount(rows: Row[], columns: Column<Row>[], narrowedBy = "") {
+  host = document.createElement("div");
+  document.body.appendChild(host);
+  root = createRoot(host);
+  const draw = (r: Row[], n: string) =>
+    flushSync(() =>
+      root!.render(
+        <DataTable rows={r} columns={columns} rowKey={(x) => x.id} pageSize={0} narrowedBy={n} />,
+      ),
+    );
+  draw(rows, narrowedBy);
+  return draw;
+}
+
+const plain: Column<Row>[] = [{ key: "name", header: "Name", render: (r) => r.name }];
+
+describe("DataTable — a long page mounts in steps", () => {
+  it("paints its first rows at once and the rest after", async () => {
+    mount(many(200), plain);
+    expect(bodyRows()).toBe(FIRST_ROWS);
+    await waitFor(() => expect(bodyRows()).toBe(200));
+  });
+
+  it("does not re-render the rows it already holds as it grows", async () => {
+    const renders = new Map<string, number>();
+    const counted: Column<Row>[] = [
+      {
+        key: "name",
+        header: "Name",
+        render: (r) => {
+          renders.set(r.id, (renders.get(r.id) ?? 0) + 1);
+          return r.name;
+        },
+      },
+    ];
+    mount(many(200), counted);
+    await waitFor(() => expect(bodyRows()).toBe(200));
+    // The first row was drawn by the first render, not again by each step after it.
+    expect(renders.get("r0")).toBe(1);
+  });
+
+  it("starts over from the first rows when the caller narrows them, not on new data", async () => {
+    const draw = mount(many(200), plain, "");
+    await waitFor(() => expect(bodyRows()).toBe(200));
+    // New data for the same view (a refetch): the page keeps what it shows.
+    draw(many(200), "");
+    expect(bodyRows()).toBe(200);
+    // A search the caller applied: the urgent render is small again.
+    draw(many(150, "s"), "s");
+    expect(bodyRows()).toBe(FIRST_ROWS);
+    await waitFor(() => expect(bodyRows()).toBe(150));
+  });
+});

frontend/src/components/ui/DataTable.tsx

@@ -11,6 +11,7 @@ import { navItemAttrs, useListNav } from "../../lib/listNav";
 import { useMediaQuery } from "../../lib/useMediaQuery";
 import { SURFACE } from "./surfaces";
 import { HiddenFilterCue } from "./HiddenFilterCue";
+import { useProgressiveRows } from "./progressiveRows";
 
 // A reusable, client-side data table: per-column sort + filter (in the header) + pagination,
 // with its sort/filter/page state optionally persisted to localStorage so a reload (F5) keeps
@@ -48,6 +49,45 @@ function loadPersist(key?: string): Persisted {
   }
 }
 
+interface TableRowProps<T> {
+  row: T;
+  index: number;
+  columns: Column<T>[];
+  rowProps?: (row: T) => Record<string, unknown>;
+  rowClassName?: (row: T) => string;
+  keyboardNav: boolean;
+}
+
+// A row is its own component so the React Compiler caches its cells by (row, columns, …): a long page
+// grows in steps (progressiveRows), and a step must not draw the rows it already holds again. Inline in
+// the body's map they all were, and the steps grew from ~70 to ~200 ms (dev build) as the page filled.
+function TableRow<T>({
+  row,
+  index,
+  columns,
+  rowProps,
+  rowClassName,
+  keyboardNav,
+}: TableRowProps<T>) {
+  return (
+    <tr
+      {...rowProps?.(row)}
+      {...(keyboardNav && navItemAttrs(index))}
+      className={`border-b border-border/50 state-layer transition ${rowClassName?.(row) ?? ""}`}
+    >
+      {columns.map((col) => (
+        <td
+          key={col.key}
+          style={col.width ? { width: col.width } : undefined}
+          className={`px-2 py-2 align-middle ${col.nowrap ? "whitespace-nowrap" : ""} ${alignClass(col.align)}${columnHideClass(col)}`}
+        >
+          {col.render(row)}
+        </td>
+      ))}
+    </tr>
+  );
+}
+
 export default function DataTable<T>({
   rows,
   columns,
@@ -72,6 +112,7 @@ export default function DataTable<T>({
   keyboardNav = false,
   reveal,
   surfacedFilters,
+  narrowedBy = "",
 }: {
   rows: T[];
   columns: Column<T>[];
@@ -131,6 +172,10 @@ export default function DataTable<T>({
   // Filter keys the caller shows somewhere besides their column (the channel manager's tag chips):
   // the hidden-filter cue leaves those out, it would name them twice.
   surfacedFilters?: string[];
+  // What the caller narrowed `rows` by before passing them in (the channel manager's name search lives
+  // in the shared top bar): a change starts a long page over from its first rows, as the table's own
+  // sort and filter changes do (progressiveRows). Without it every key re-rendered the whole page.
+  narrowedBy?: string;
 }) {
   const { t, i18n } = useTranslation();
   const lang = i18n.language;
@@ -229,6 +274,11 @@ export default function DataTable<T>({
     if (at >= 0 && !allRows) setPage(Math.floor(at / size));
   }
   const paged = allRows ? sorted : sorted.slice(safePage * size, safePage * size + size);
+  // A long page ("All", 100) mounts its first rows at once and the rest in the background.
+  const shown = useProgressiveRows(
+    paged,
+    `${safePage}:${size}:${JSON.stringify(sort)}:${JSON.stringify(filters)}:${narrowedBy}`,
+  );
 
   function toggleSort(key: string) {
     const next = cycleSort(sort, key);
@@ -327,25 +377,16 @@ export default function DataTable<T>({
               onFilter={applyFilter}
             />
             <tbody ref={bodyRef}>
-              {paged.map((row, i) => (
-                <tr
+              {shown.map((row, i) => (
+                <TableRow
                   key={rowKey(row)}
-                  {...rowProps?.(row)}
-                  {...(keyboardNav && navItemAttrs(i))}
-                  className={`border-b border-border/50 state-layer transition ${
-                    rowClassName?.(row) ?? ""
-                  }`}
-                >
-                  {columns.map((col) => (
-                    <td
-                      key={col.key}
-                      style={col.width ? { width: col.width } : undefined}
-                      className={`px-2 py-2 align-middle ${col.nowrap ? "whitespace-nowrap" : ""} ${alignClass(col.align)}${columnHideClass(col)}`}
-                    >
-                      {col.render(row)}
-                    </td>
-                  ))}
-                </tr>
+                  row={row}
+                  index={i}
+                  columns={columns}
+                  rowProps={rowProps}
+                  rowClassName={rowClassName}
+                  keyboardNav={keyboardNav}
+                />
               ))}
             </tbody>
           </table>
@@ -356,7 +397,7 @@ export default function DataTable<T>({
         <>
           {cue("cards")}
           <div ref={cardsRef} className="flex flex-col gap-2">
-            {paged.map((row, i) => (
+            {shown.map((row, i) => (
               <div
                 key={rowKey(row)}
                 {...rowProps?.(row)}

frontend/src/components/ui/progressiveRows.ts

@@ -0,0 +1,33 @@
+import { startTransition, useEffect, useState } from "react";
+
+// A long page of table rows mounts in steps: the first FIRST_ROWS at once, then STEP_ROWS more per
+// background transition until all are in (E31 S4d). The channel manager's "All" page (~400 rows) took
+// 0.7–0.9 s of one blocked frame to mount on prod, and every sort and every keystroke in its search
+// re-rendered all of them (300–400 ms per key). No single cell is to blame: measured column by column,
+// each costs its share, and all of them together cost ~9× an empty row. The first rows fill more than
+// a screen, so the page paints at once. The rest come in small steps because a transition yields only
+// while it renders: its commit (the rows' DOM, the tag cells' measuring layout effects) is one block,
+// and the rest in one step was a 0.7 s block of its own. Between steps a key or a click goes first,
+// and a new view starts over.
+//
+// Not virtualization: every row still ends up in the DOM, so browser find, keyboard nav past the screen,
+// and the Back focus/scroll restore (which wait for the list to grow) keep working as they did.
+export const FIRST_ROWS = 30;
+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[] {
+  const [seen, setSeen] = useState(view);
+  const [limit, setLimit] = useState(FIRST_ROWS);
+  if (seen !== view) {
+    setSeen(view);
+    setLimit(FIRST_ROWS);
+  }
+  const more = rows.length > limit;
+  useEffect(() => {
+    if (more) startTransition(() => setLimit(limit + STEP_ROWS));
+  }, [more, limit]);
+  return more ? rows.slice(0, limit) : rows;
+}