shared page: fix expand freeze — cache date formatter, memoize rows

Expanding a receipt on /shared froze the browser: this table renders every
split row at once (1,279 today), and one state change re-rendered all of
them. 520ms of each pass was formatDate constructing a fresh
Intl.DateTimeFormat per call (2,558 calls per render); the rest was
rebuilding 1,279 rows to change one. Headless-measured click stall:
802ms -> 165ms on the server; slower machines multiply the former.

- module-level DATE_FMT, reused
- rows extracted into memoized SharedTxRow with stable callbacks
- filter+sort wrapped in useMemo so unrelated renders skip it

src/app/shared/page.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { Fragment, useState, useRef, useEffect } from "react";
+import { Fragment, memo, useCallback, useMemo, useState, useRef, useEffect } from "react";
 import {
   useSharedTransactions,
   useParticipantBalances,
@@ -20,8 +20,12 @@ import { OrderDetails } from "@/components/order-details";
 import { formatCategory } from "@/lib/categories";
 import { CATEGORY_COLORS } from "@/lib/category-colors";
 
+// One formatter, reused. `toLocaleDateString` constructs a fresh
+// Intl.DateTimeFormat per call, and this page renders every split row at once —
+// 2,558 calls per render was 520ms of the expand-click freeze on its own.
+const DATE_FMT = new Intl.DateTimeFormat("en-AU", { day: "numeric", month: "short", year: "numeric" });
 function formatDate(d: string) {
-  return new Date(d).toLocaleDateString("en-AU", { day: "numeric", month: "short", year: "numeric" });
+  return DATE_FMT.format(new Date(d));
 }
 
 const SPEND_TYPES = new Set(["debit", "fee", "interest"]);
@@ -306,6 +310,125 @@ function PaymentHistory({ participantId, currentUserId }: { participantId: numbe
   );
 }
 
+// ── Split transaction row ─────────────────────────────────────────────────────
+// Memoized: this table renders every split row at once (1,279 today, no
+// pagination), so any state change on the page — expanding a receipt, each of
+// its two query results arriving — re-rendered all of them, ~800ms per pass on
+// a fast machine and a multi-second browser freeze on a slow one. With memo,
+// only the row whose `isExpanded` flipped re-renders. Every prop must stay
+// referentially stable: `tx` objects come straight out of the query cache, and
+// the callbacks are a state setter and a useCallback.
+const SharedTxRow = memo(function SharedTxRow({
+  tx,
+  isExpanded,
+  meId,
+  onToggle,
+  onEdit,
+}: {
+  tx: SharedTransactionRow;
+  isExpanded: boolean;
+  meId: number | undefined;
+  onToggle: (id: number) => void;
+  onEdit: (tx: SharedTransactionRow) => void;
+}) {
+  const splits = Array.isArray(tx.splits) ? tx.splits : [];
+  return (
+    <Fragment>
+      <tr className="border-b border-zinc-800/50 hover:bg-zinc-800/30">
+        <td className="px-4 py-3 text-zinc-400 whitespace-nowrap">{formatDate(tx.transaction_date)}</td>
+        <td className="px-4 py-3 text-zinc-500 text-xs whitespace-nowrap">{formatDate(tx.created_at)}</td>
+        <td className="px-4 py-3 max-w-xs sticky left-0 z-10 bg-zinc-900 border-r border-zinc-800/80">
+          <div className="flex items-start gap-1.5">
+            {tx.order_platform && (
+              <button
+                onClick={() => onToggle(tx.id)}
+                className="text-zinc-600 hover:text-zinc-300 leading-none mt-0.5 shrink-0"
+                title={isExpanded ? "Hide receipt" : "Show the receipt this came from"}
+                aria-expanded={isExpanded}
+              >
+                {isExpanded ? "▾" : "▸"}
+              </button>
+            )}
+            <p className="font-medium break-words">{tx.effective_merchant || tx.description}</p>
+          </div>
+          {tx.effective_merchant && (
+            <p className="text-xs text-zinc-500 break-words">{tx.description}</p>
+          )}
+          {tx.notes && (
+            <p className="text-xs text-zinc-500 italic mt-0.5 break-words">{tx.notes}</p>
+          )}
+        </td>
+        {/* Category is the effective one — the override wins over the
+            extracted value, the same COALESCE every other view uses,
+            so a correction made elsewhere shows up here too. */}
+        <td className="px-4 py-3 whitespace-nowrap">
+          {tx.effective_category ? (
+            <span
+              className="inline-flex items-center gap-1.5 text-xs text-zinc-300"
+              title={formatCategory(tx.effective_category)}
+            >
+              <span
+                className="w-1.5 h-1.5 rounded-full flex-shrink-0"
+                style={{ background: CATEGORY_COLORS[tx.effective_category] ?? "#71717a" }}
+              />
+              {formatCategory(tx.effective_category)}
+            </span>
+          ) : (
+            <span className="text-xs text-zinc-600 italic">uncategorised</span>
+          )}
+        </td>
+        <td className={`px-4 py-3 text-right font-medium tabular-nums ${SPEND_TYPES.has(tx.transaction_type) ? "" : "text-green-400"}`}>
+          {formatAmount(tx.amount, tx.transaction_type, tx.currency)}
+          {tx.currency !== "AUD" && (
+            // Splits settle on the AUD figure, so show it next to the
+            // native one rather than leaving the two to differ silently.
+            <span className="block text-xs font-normal text-zinc-500">
+              {tx.amount_unconverted
+                ? "AUD value unknown"
+                : `≈ ${formatAmount(Number(tx.amount_aud), tx.transaction_type, "AUD")} AUD`}
+            </span>
+          )}
+        </td>
+        {/* Whose money actually left. This is the effective owner —
+            COALESCE(t.owner_id, s.owner_id) — so it is the account the
+            spend came out of, which is what every balance on this page
+            is computed from. "Me" matches the split chips rather than
+            printing your own name twice in one row. */}
+        <td className="px-4 py-3 whitespace-nowrap">
+          <span className={`text-xs ${tx.owner_id === meId ? "text-zinc-400" : "text-indigo-300"}`}>
+            {tx.owner_id === meId ? "Me" : tx.owner_name}
+          </span>
+        </td>
+        <td className="px-4 py-3">
+          <div className="flex flex-wrap gap-1">
+            {splits.map((s) => (
+              <span key={s.participant_id}
+                className="inline-flex items-center gap-1 px-2 py-0.5 rounded text-xs bg-zinc-800 text-zinc-300">
+                {s.participant_id === meId ? "Me" : s.name} {s.share_percent}%
+              </span>
+            ))}
+          </div>
+        </td>
+        <td className="px-4 py-3">
+          <button
+            onClick={() => onEdit(tx)}
+            className="text-xs text-zinc-500 hover:text-zinc-200 px-2 py-0.5 rounded hover:bg-zinc-800 transition-colors"
+          >
+            Edit
+          </button>
+        </td>
+      </tr>
+      {isExpanded && (
+        <tr className="border-b border-zinc-800/50 bg-zinc-900/40">
+          <td colSpan={8} className="px-4 py-3">
+            <OrderDetails transactionId={tx.id} currency={tx.currency ?? null} bare />
+          </td>
+        </tr>
+      )}
+    </Fragment>
+  );
+});
+
 // ── Main page ─────────────────────────────────────────────────────────────────
 type SortCol = "transaction_date" | "created_at" | "amount";
 
@@ -328,7 +451,9 @@ export default function SharedPage() {
   // already does that properly, and typing "sonu" matching every row she is
   // split on would make the box look broken. The payer IS matched, because
   // nothing else on the page filters by who paid.
-  const transactions = [...rawTransactions]
+  const transactions = useMemo(
+    () =>
+      [...rawTransactions]
         .filter((tx) => {
           const q = search.trim().toLowerCase();
           if (!q) return true;
@@ -344,7 +469,9 @@ export default function SharedPage() {
           const av = sortCol === "amount" ? Number(a.amount) : new Date(a[sortCol]).getTime();
           const bv = sortCol === "amount" ? Number(b.amount) : new Date(b[sortCol]).getTime();
           return sortDir === "desc" ? bv - av : av - bv;
-    });
+        }),
+    [rawTransactions, search, sortCol, sortDir]
+  );
 
   function toggleSort(col: SortCol) {
     if (sortCol === col) setSortDir((d) => (d === "desc" ? "asc" : "desc"));
@@ -375,6 +502,15 @@ export default function SharedPage() {
   // viewer is a split participant, so /api/transactions/[id]/order
   // authorises them — the item list is part of what was shared.
   const [expanded, setExpanded] = useState<Set<number>>(new Set());
+  // Stable identity so SharedTxRow's memo holds — an inline closure here would
+  // change every render and re-render all 1,279 rows anyway.
+  const toggleExpanded = useCallback((id: number) => {
+    setExpanded((prev) => {
+      const next = new Set(prev);
+      if (next.has(id)) next.delete(id); else next.add(id);
+      return next;
+    });
+  }, []);
 
   return (
     <div className="space-y-6">
@@ -545,108 +681,16 @@ export default function SharedPage() {
               </tr>
             </thead>
             <tbody>
-              {(transactions as SharedTransactionRow[]).map((tx) => {
-                const splits = Array.isArray(tx.splits) ? tx.splits : [];
-                return (
-                  <Fragment key={tx.id}>
-                  <tr className="border-b border-zinc-800/50 hover:bg-zinc-800/30">
-                    <td className="px-4 py-3 text-zinc-400 whitespace-nowrap">{formatDate(tx.transaction_date)}</td>
-                    <td className="px-4 py-3 text-zinc-500 text-xs whitespace-nowrap">{formatDate(tx.created_at)}</td>
-                    <td className="px-4 py-3 max-w-xs sticky left-0 z-10 bg-zinc-900 border-r border-zinc-800/80">
-                      <div className="flex items-start gap-1.5">
-                        {tx.order_platform && (
-                          <button
-                            onClick={() => setExpanded((prev) => {
-                              const next = new Set(prev);
-                              if (next.has(tx.id)) next.delete(tx.id); else next.add(tx.id);
-                              return next;
-                            })}
-                            className="text-zinc-600 hover:text-zinc-300 leading-none mt-0.5 shrink-0"
-                            title={expanded.has(tx.id) ? "Hide receipt" : "Show the receipt this came from"}
-                            aria-expanded={expanded.has(tx.id)}
-                          >
-                            {expanded.has(tx.id) ? "▾" : "▸"}
-                          </button>
-                        )}
-                        <p className="font-medium break-words">{tx.effective_merchant || tx.description}</p>
-                      </div>
-                      {tx.effective_merchant && (
-                        <p className="text-xs text-zinc-500 break-words">{tx.description}</p>
-                      )}
-                      {tx.notes && (
-                        <p className="text-xs text-zinc-500 italic mt-0.5 break-words">{tx.notes}</p>
-                      )}
-                    </td>
-                    {/* Category is the effective one — the override wins over the
-                        extracted value, the same COALESCE every other view uses,
-                        so a correction made elsewhere shows up here too. */}
-                    <td className="px-4 py-3 whitespace-nowrap">
-                      {tx.effective_category ? (
-                        <span
-                          className="inline-flex items-center gap-1.5 text-xs text-zinc-300"
-                          title={formatCategory(tx.effective_category)}
-                        >
-                          <span
-                            className="w-1.5 h-1.5 rounded-full flex-shrink-0"
-                            style={{ background: CATEGORY_COLORS[tx.effective_category] ?? "#71717a" }}
+              {(transactions as SharedTransactionRow[]).map((tx) => (
+                <SharedTxRow
+                  key={tx.id}
+                  tx={tx}
+                  isExpanded={expanded.has(tx.id)}
+                  meId={me?.id}
+                  onToggle={toggleExpanded}
+                  onEdit={setEditModal}
                 />
-                          {formatCategory(tx.effective_category)}
-                        </span>
-                      ) : (
-                        <span className="text-xs text-zinc-600 italic">uncategorised</span>
-                      )}
-                    </td>
-                    <td className={`px-4 py-3 text-right font-medium tabular-nums ${SPEND_TYPES.has(tx.transaction_type) ? "" : "text-green-400"}`}>
-                      {formatAmount(tx.amount, tx.transaction_type, tx.currency)}
-                      {tx.currency !== "AUD" && (
-                        // Splits settle on the AUD figure, so show it next to the
-                        // native one rather than leaving the two to differ silently.
-                        <span className="block text-xs font-normal text-zinc-500">
-                          {tx.amount_unconverted
-                            ? "AUD value unknown"
-                            : `≈ ${formatAmount(Number(tx.amount_aud), tx.transaction_type, "AUD")} AUD`}
-                        </span>
-                      )}
-                    </td>
-                    {/* Whose money actually left. This is the effective owner —
-                        COALESCE(t.owner_id, s.owner_id) — so it is the account the
-                        spend came out of, which is what every balance on this page
-                        is computed from. "Me" matches the split chips rather than
-                        printing your own name twice in one row. */}
-                    <td className="px-4 py-3 whitespace-nowrap">
-                      <span className={`text-xs ${tx.owner_id === me?.id ? "text-zinc-400" : "text-indigo-300"}`}>
-                        {tx.owner_id === me?.id ? "Me" : tx.owner_name}
-                      </span>
-                    </td>
-                    <td className="px-4 py-3">
-                      <div className="flex flex-wrap gap-1">
-                        {splits.map((s) => (
-                          <span key={s.participant_id}
-                            className="inline-flex items-center gap-1 px-2 py-0.5 rounded text-xs bg-zinc-800 text-zinc-300">
-                            {s.participant_id === me?.id ? "Me" : s.name} {s.share_percent}%
-                          </span>
               ))}
-                      </div>
-                    </td>
-                    <td className="px-4 py-3">
-                      <button
-                        onClick={() => setEditModal(tx)}
-                        className="text-xs text-zinc-500 hover:text-zinc-200 px-2 py-0.5 rounded hover:bg-zinc-800 transition-colors"
-                      >
-                        Edit
-                      </button>
-                    </td>
-                  </tr>
-                  {expanded.has(tx.id) && (
-                    <tr className="border-b border-zinc-800/50 bg-zinc-900/40">
-                      <td colSpan={8} className="px-4 py-3">
-                        <OrderDetails transactionId={tx.id} currency={tx.currency ?? null} bare />
-                      </td>
-                    </tr>
-                  )}
-                  </Fragment>
-                );
-              })}
             </tbody>
           </table>
         )}