fix(#370): ES2021-safe spec, source-specific idle ceiling, processor lock, tested index mapping (review round 1)

F1 [BLOCKER] persistence-store.spec used Array.prototype.at(-1) (ES2022) but the
server targets ES2021, so server tsc failed (TS2550) and ts-jest could not
compile the suite — 22 core manual-save/idle/boundary tests silently did not run
in CI. Replaced with [length - 1] index access.

F2 [WARNING] The idle burst-reset used a hardcoded IDLE_MAX_WAIT_USER for both
tiers, but computeHistoryJob's ceiling is source-specific. On a continuously
agent-edited page the burst marker stayed stale for 5..10m, forcing delay=0 on
every store and writing one idle row per store — the exact per-store bloat the
debounce prevents. The reset now uses the same source-specific max-wait.

F3 [WARNING] The processor did an unlocked findPageLastHistory -> saveHistory,
which TOCTOU-races a concurrent manual-save (that runs under a page-row lock),
producing two page_history rows with identical content (one idle, one manual) and
defeating promote-not-dup. The snapshot decision is now wrapped in executeTx with
the same page-row lock, so the second writer observes the first's committed row
and the isDeepStrictEqual gate collapses the duplicate.

F4 [WARNING] The risky client filtered-index -> full-list mapping had no tests.
Extracted it to a pure resolvePrevSnapshotId(fullItems, id) helper (diff/restore
baseline against the true previous snapshot in the FULL list, never the previous
visible version) and unit-tested it; removed the now-vestigial index threading.

F5/F6 [low] Renamed the misleading ceiling test + fixed its comment; added a
CHANGELOG entry for the user-facing versioning feature.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/page-history/components/history-item.tsx

@@ -40,16 +40,17 @@ export function historyKindMeta(kind?: string | null): HistoryKindMeta {
 
 interface HistoryItemProps {
   historyItem: IPageHistory;
-  index: number;
-  onSelect: (id: string, index: number) => void;
-  onHover?: (id: string, index: number) => void;
+  // The previous snapshot for diff/restore is resolved by id from the FULL list
+  // in the parent (resolvePrevSnapshotId), so the item only needs to report its
+  // own id — never a list index (which would be the filtered-view index).
+  onSelect: (id: string) => void;
+  onHover?: (id: string) => void;
   onHoverEnd?: () => void;
   isActive: boolean;
 }
 
 const HistoryItem = memo(function HistoryItem({
   historyItem,
-  index,
   onSelect,
   onHover,
   onHoverEnd,
@@ -60,12 +61,12 @@ const HistoryItem = memo(function HistoryItem({
   const kindMeta = historyKindMeta(historyItem.kind);
 
   const handleClick = useCallback(() => {
-    onSelect(historyItem.id, index);
-  }, [onSelect, historyItem.id, index]);
+    onSelect(historyItem.id);
+  }, [onSelect, historyItem.id]);
 
   const handleMouseEnter = useCallback(() => {
-    onHover?.(historyItem.id, index);
-  }, [onHover, historyItem.id, index]);
+    onHover?.(historyItem.id);
+  }, [onHover, historyItem.id]);
 
   const contributors = historyItem.contributors;
   const hasContributors = contributors && contributors.length > 0;

apps/client/src/features/page-history/components/history-list.tsx

@@ -22,6 +22,7 @@ import {
 } from "@mantine/core";
 import { useTranslation } from "react-i18next";
 import { useHistoryRestore } from "@/features/page-history/hooks";
+import { resolvePrevSnapshotId } from "@/features/page-history/utils/resolve-prev-snapshot";
 
 const PREFETCH_DELAY_MS = 150;
 
@@ -58,11 +59,6 @@ function HistoryList({ pageId }: Props) {
     (kind?: string | null) => kind === "manual" || kind === "agent",
     [],
   );
-  const originalIndexById = useMemo(() => {
-    const map = new Map<string, number>();
-    historyItems.forEach((item, index) => map.set(item.id, index));
-    return map;
-  }, [historyItems]);
   const visibleItems = useMemo(
     () =>
       onlyVersions
@@ -84,11 +80,13 @@ function HistoryList({ pageId }: Props) {
   }, []);
 
   const handleHover = useCallback(
-    (historyId: string, index: number) => {
+    (historyId: string) => {
       clearPrefetchTimeout();
       prefetchTimeoutRef.current = setTimeout(() => {
         prefetchPageHistory(historyId);
-        const prevId = historyItems[index + 1]?.id;
+        // The true previous snapshot in the FULL list (not the previous visible
+        // one under the "only versions" filter).
+        const prevId = resolvePrevSnapshotId(historyItems, historyId);
         if (prevId) {
           prefetchPageHistory(prevId);
         }
@@ -102,9 +100,11 @@ function HistoryList({ pageId }: Props) {
   }, [clearPrefetchTimeout]);
 
   const handleSelect = useCallback(
-    (id: string, index: number) => {
+    (id: string) => {
       setActiveHistoryId(id);
-      setActiveHistoryPrevId(historyItems[index + 1]?.id ?? "");
+      // Baseline = true previous snapshot in the FULL list, so the "only
+      // versions" filter never diffs/restores against the wrong item.
+      setActiveHistoryPrevId(resolvePrevSnapshotId(historyItems, id));
     },
     [historyItems, setActiveHistoryId, setActiveHistoryPrevId],
   );
@@ -173,9 +173,6 @@ function HistoryList({ pageId }: Props) {
           <HistoryItem
             key={historyItem.id}
             historyItem={historyItem}
-            // Index within the FULL list so diff/restore uses the true previous
-            // snapshot, not the previous visible one.
-            index={originalIndexById.get(historyItem.id) ?? 0}
             onSelect={handleSelect}
             onHover={handleHover}
             onHoverEnd={clearPrefetchTimeout}

apps/client/src/features/page-history/utils/resolve-prev-snapshot.test.ts

@@ -0,0 +1,42 @@
+import { describe, it, expect } from "vitest";
+import { resolvePrevSnapshotId } from "./resolve-prev-snapshot";
+
+// #370 F4 — the risky client path: with the "only versions" filter active, diff
+// and restore must still baseline against the TRUE previous snapshot in the FULL
+// list, never the previous VISIBLE version (which would skip the autosnapshots
+// between two versions). These pin that the resolution is by FULL-list order.
+describe("resolvePrevSnapshotId", () => {
+  // Newest-first, as the history list stores it: a version, then two autosaves,
+  // then an older version.
+  const full = [
+    { id: "v2", kind: "manual" },
+    { id: "a2", kind: "idle" },
+    { id: "a1", kind: "boundary" },
+    { id: "v1", kind: "manual" },
+    { id: "a0", kind: null },
+  ];
+
+  it("returns the immediate FULL-list successor, not the previous visible version", () => {
+    // Selecting v2 while filtered to versions-only must baseline against a2 (the
+    // real chronological predecessor), NOT v1 (the previous visible version).
+    expect(resolvePrevSnapshotId(full, "v2")).toBe("a2");
+  });
+
+  it("resolves an autosnapshot's predecessor by full-list order", () => {
+    expect(resolvePrevSnapshotId(full, "a1")).toBe("v1");
+  });
+
+  it("returns '' for the oldest item (no predecessor)", () => {
+    expect(resolvePrevSnapshotId(full, "a0")).toBe("");
+  });
+
+  it("returns '' for an id not in the list", () => {
+    expect(resolvePrevSnapshotId(full, "missing")).toBe("");
+  });
+
+  it("does not depend on a filtered subset — same result whatever is visible", () => {
+    // The helper only ever sees the full list; a filtered view cannot change the
+    // baseline it computes.
+    expect(resolvePrevSnapshotId(full, "v1")).toBe("a0");
+  });
+});

apps/client/src/features/page-history/utils/resolve-prev-snapshot.ts

@@ -0,0 +1,22 @@
+/**
+ * #370 — resolve the TRUE previous snapshot for a history item.
+ *
+ * The history panel can be filtered to "only versions" (manual/agent), but diff
+ * and restore must always compare against the immediately-preceding snapshot in
+ * the FULL, unfiltered list — NOT the previous VISIBLE item. Comparing against
+ * the previous visible version would silently skip the autosnapshots between two
+ * versions and diff/restore the wrong baseline.
+ *
+ * Given the full (newest-first) list and an item id, this returns the id of the
+ * item right after it in the full list (its chronological predecessor), or "" if
+ * it is the oldest / not found. Pure and list-order-preserving so it can be unit
+ * tested without mounting the component.
+ */
+export function resolvePrevSnapshotId(
+  fullItems: ReadonlyArray<{ id: string }>,
+  id: string,
+): string {
+  const index = fullItems.findIndex((item) => item.id === id);
+  if (index === -1) return "";
+  return fullItems[index + 1]?.id ?? "";
+}