perf(inbox): virtualize notification list (MUL-4474)

The inbox notification list rendered every item at once. Each row mounts an
avatar + hover card, so a long inbox inflates the tab-switch commit — the
same render-amplifier class this issue targets.

Extract an InboxList component that virtualizes the rows via react-virtuoso
(customScrollParent over the existing overflow-y-auto element, same pattern
as the issue-detail timeline). Only the visible window plus a small overscan
is mounted; everything else — selection, hover, archive, scroll semantics,
the row component and callbacks — is unchanged. Virtualization changes
exactly one thing: whether an off-screen row is in the DOM.

Slice 2a of MUL-4474 (inbox is the no-DnD surface, done first to prove the
Virtuoso + scroll + keyboard harness before the drag surfaces). Draft: must
pass the manual zero-functional-change pass on a real Desktop build before
merge.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: multica-agent <github@multica.ai>

packages/views/inbox/components/inbox-list.tsx

@@ -0,0 +1,81 @@
+"use client";
+
+import { useState } from "react";
+import { Virtuoso } from "react-virtuoso";
+import { Inbox } from "lucide-react";
+import type { InboxItem } from "@multica/core/types";
+import { InboxListItem } from "./inbox-list-item";
+import { useT } from "../../i18n";
+
+/**
+ * Scrollable, virtualized inbox notification list.
+ *
+ * Owns the scroll container so both the mobile and desktop layouts render an
+ * identical scroller. Rows are virtualized via react-virtuoso so only the
+ * visible window (plus a small overscan) is mounted — the notification list
+ * can grow long and every row otherwise carries an avatar + hover card, so
+ * mounting all of them inflates the tab-switch commit (MUL-4474).
+ *
+ * Virtualization changes exactly one thing: whether an off-screen row is in
+ * the DOM. Selection, hover, archive, and scroll semantics are unchanged —
+ * the row component and the callbacks are the same as the non-virtualized
+ * list. `customScrollParent` keeps Virtuoso reading/writing the existing
+ * `overflow-y-auto` element (same pattern as the issue-detail timeline), so
+ * scroll position behaves exactly as before.
+ *
+ * Known virtualization tradeoff: keyboard Tab only reaches currently-mounted
+ * rows; a keyboard-only user must scroll to bring off-screen rows into the
+ * tab order. The inbox has no custom arrow-key list navigation, so the
+ * practical surface is small, but it is called out for the manual pass.
+ */
+export function InboxList({
+  items,
+  selectedKey,
+  onSelect,
+  onArchive,
+}: {
+  items: InboxItem[];
+  selectedKey: string;
+  onSelect: (item: InboxItem) => void;
+  onArchive: (id: string) => void;
+}) {
+  const { t } = useT("inbox");
+  // Virtuoso's `customScrollParent` wants the actual HTMLElement, not a ref.
+  // A callback ref into state hands the element over once it mounts and
+  // triggers the re-render that lets Virtuoso attach to it.
+  const [scrollEl, setScrollEl] = useState<HTMLDivElement | null>(null);
+
+  if (items.length === 0) {
+    return (
+      <div className="flex-1 min-h-0 overflow-y-auto">
+        <div className="flex flex-col items-center justify-center py-16 text-muted-foreground">
+          <Inbox className="mb-3 h-8 w-8 text-muted-foreground/50" />
+          <p className="text-sm">{t(($) => $.list.empty)}</p>
+        </div>
+      </div>
+    );
+  }
+
+  return (
+    <div ref={setScrollEl} className="flex-1 min-h-0 overflow-y-auto">
+      <div className="px-2 py-1">
+        {scrollEl && (
+          <Virtuoso
+            customScrollParent={scrollEl}
+            data={items}
+            computeItemKey={(_index, item) => item.id}
+            increaseViewportBy={{ top: 400, bottom: 400 }}
+            itemContent={(_index, item) => (
+              <InboxListItem
+                item={item}
+                isSelected={(item.issue_id ?? item.id) === selectedKey}
+                onClick={() => onSelect(item)}
+                onArchive={() => onArchive(item.id)}
+              />
+            )}
+          />
+        )}
+      </div>
+    </div>
+  );
+}

packages/views/inbox/components/inbox-page.tsx

@@ -52,7 +52,8 @@ import {
 } from "@multica/ui/components/ui/dropdown-menu";
 import { useIsMobile } from "@multica/ui/hooks/use-mobile";
 import { PageHeader } from "../../layout/page-header";
-import { InboxListItem, useTimeAgo } from "./inbox-list-item";
+import { useTimeAgo } from "./inbox-list-item";
+import { InboxList } from "./inbox-list";
 import { useTypeLabels } from "./inbox-detail-label";
 import { getInboxDisplayTitle } from "./inbox-display";
 import { useT } from "../../i18n";
@@ -271,23 +272,13 @@ export function InboxPage() {
     </PageHeader>
   );
 
-  const listBody = items.length === 0 ? (
-    <div className="flex flex-col items-center justify-center py-16 text-muted-foreground">
-      <Inbox className="mb-3 h-8 w-8 text-muted-foreground/50" />
-      <p className="text-sm">{t(($) => $.list.empty)}</p>
-    </div>
-  ) : (
-    <div className="px-2 py-1">
-      {items.map((item) => (
-        <InboxListItem
-          key={item.id}
-          item={item}
-          isSelected={(item.issue_id ?? item.id) === selectedKey}
-          onClick={() => handleSelect(item)}
-          onArchive={() => handleArchive(item.id)}
-        />
-      ))}
-    </div>
+  const list = (
+    <InboxList
+      items={items}
+      selectedKey={selectedKey}
+      onSelect={handleSelect}
+      onArchive={handleArchive}
+    />
   );
 
   const detailContent = selected?.issue_id ? (
@@ -418,9 +409,7 @@ export function InboxPage() {
     return (
       <div className="flex flex-1 flex-col min-h-0">
         {listHeader}
-        <div className="flex-1 min-h-0 overflow-y-auto">
-          {listBody}
-        </div>
+        {list}
       </div>
     );
   }
@@ -464,9 +453,7 @@ export function InboxPage() {
       <ResizablePanel id="list" defaultSize={320} minSize={240} maxSize={480} groupResizeBehavior="preserve-pixel-size">
       <div className="flex flex-col border-r h-full">
         {listHeader}
-        <div className="flex-1 min-h-0 overflow-y-auto">
-          {listBody}
-        </div>
+        {list}
       </div>
       </ResizablePanel>
       <ResizableHandle />