concurrency

backend/src/__tests__/imports-compat.integration.ts

@@ -3,6 +3,7 @@ import request from "supertest";
 import fs from "fs";
 import path from "path";
 import os from "os";
+import JSZip from "jszip";
 import { getTestPrisma, setupTestDb, cleanupTestDb } from "./testUtils";
 
 type LegacyDbOptions = {
@@ -156,6 +157,111 @@ const createLegacySqliteDb = (opts: LegacyDbOptions): string => {
   return filePath;
 };
 
+const createExcalidashArchiveWithDuplicateDrawingIds = async (): Promise<string> => {
+  const dir = createTempDir();
+  const filePath = path.join(dir, "duplicate-drawing-ids.excalidash");
+  const zip = new JSZip();
+
+  const manifest = {
+    format: "excalidash",
+    formatVersion: 1,
+    exportedAt: new Date().toISOString(),
+    unorganizedFolder: "Unorganized",
+    collections: [] as any[],
+    drawings: [
+      {
+        id: "duplicate-drawing-id",
+        name: "Drawing One",
+        filePath: "Unorganized/drawing-1.excalidraw",
+        collectionId: null,
+      },
+      {
+        id: "duplicate-drawing-id",
+        name: "Drawing Two",
+        filePath: "Unorganized/drawing-2.excalidraw",
+        collectionId: null,
+      },
+    ],
+  };
+
+  zip.file("excalidash.manifest.json", JSON.stringify(manifest));
+  zip.file(
+    "Unorganized/drawing-1.excalidraw",
+    JSON.stringify({ type: "excalidraw", version: 2, source: "test", elements: [], appState: {}, files: {} })
+  );
+  zip.file(
+    "Unorganized/drawing-2.excalidraw",
+    JSON.stringify({ type: "excalidraw", version: 2, source: "test", elements: [], appState: {}, files: {} })
+  );
+
+  const buffer = await zip.generateAsync({ type: "nodebuffer" });
+  fs.writeFileSync(filePath, buffer);
+  return filePath;
+};
+
+const createLegacySqliteDbWithDuplicateDrawingIds = (): string => {
+  const dir = createTempDir();
+  const filePath = path.join(dir, "legacy-duplicate-ids.db");
+  const db = openWritableDb(filePath);
+
+  try {
+    db.exec(`
+      CREATE TABLE "Drawing" (
+        id TEXT,
+        name TEXT NOT NULL,
+        elements TEXT NOT NULL,
+        appState TEXT NOT NULL,
+        files TEXT,
+        preview TEXT,
+        version INTEGER,
+        collectionId TEXT,
+        collectionName TEXT,
+        createdAt TEXT,
+        updatedAt TEXT
+      );
+    `);
+
+    const now = new Date("2024-01-03T00:00:00.000Z").toISOString();
+    const insertDrawing = db.prepare(
+      `INSERT INTO "Drawing"
+        (id, name, elements, appState, files, preview, version, collectionId, collectionName, createdAt, updatedAt)
+        VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`
+    );
+
+    insertDrawing.run(
+      "legacy-duplicate-id",
+      "Legacy Drawing A",
+      JSON.stringify([]),
+      JSON.stringify({}),
+      JSON.stringify({}),
+      null,
+      1,
+      null,
+      null,
+      now,
+      now,
+    );
+
+    insertDrawing.run(
+      "legacy-duplicate-id",
+      "Legacy Drawing B",
+      JSON.stringify([]),
+      JSON.stringify({}),
+      JSON.stringify({}),
+      null,
+      1,
+      null,
+      null,
+      now,
+      now,
+    );
+  } finally {
+    db.close();
+  }
+
+  return filePath;
+};
+
 describe("Import compatibility (legacy exports)", () => {
   const uploadsDir = path.resolve(__dirname, "../../uploads");
   const userAgent = "vitest-import-compat";
@@ -287,4 +393,52 @@ describe("Import compatibility (legacy exports)", () => {
     expect(res.status).toBe(400);
     expect(res.body.error).toBe("Invalid legacy DB");
   });
+
+  it("rejects .excalidash verify when manifest has duplicate drawing IDs", async () => {
+    const archive = await createExcalidashArchiveWithDuplicateDrawingIds();
+    const res = await request(app)
+      .post("/import/excalidash/verify")
+      .set("User-Agent", userAgent)
+      .set(csrfHeaderName, csrfToken)
+      .attach("archive", archive);
+
+    expect(res.status).toBe(400);
+    expect(String(res.body.message || "")).toContain("Duplicate drawing id");
+  });
+
+  it("rejects .excalidash import when manifest has duplicate drawing IDs", async () => {
+    const archive = await createExcalidashArchiveWithDuplicateDrawingIds();
+    const res = await request(app)
+      .post("/import/excalidash")
+      .set("User-Agent", userAgent)
+      .set(csrfHeaderName, csrfToken)
+      .attach("archive", archive);
+
+    expect(res.status).toBe(400);
+    expect(String(res.body.message || "")).toContain("Duplicate drawing id");
+  });
+
+  it("rejects legacy verify when DB has duplicate drawing IDs", async () => {
+    const legacyDb = createLegacySqliteDbWithDuplicateDrawingIds();
+    const res = await request(app)
+      .post("/import/sqlite/legacy/verify")
+      .set("User-Agent", userAgent)
+      .set(csrfHeaderName, csrfToken)
+      .attach("db", legacyDb);
+
+    expect(res.status).toBe(400);
+    expect(String(res.body.message || "")).toContain("Duplicate drawing id");
+  });
+
+  it("rejects legacy import when DB has duplicate drawing IDs", async () => {
+    const legacyDb = createLegacySqliteDbWithDuplicateDrawingIds();
+    const res = await request(app)
+      .post("/import/sqlite/legacy")
+      .set("User-Agent", userAgent)
+      .set(csrfHeaderName, csrfToken)
+      .attach("db", legacyDb);
+
+    expect(res.status).toBe(400);
+    expect(String(res.body.message || "")).toContain("Duplicate drawing id");
+  });
 });

backend/src/index.ts

@@ -560,6 +560,7 @@ const drawingUpdateSchema = drawingBaseSchema
     elements: elementSchema.array().optional(),
     appState: appStateSchema.optional(),
     files: filesFieldSchema,
+    version: z.number().int().positive().optional(),
   })
   .refine(
     (data) => {

backend/src/routes/dashboard.ts

@@ -310,7 +310,12 @@ export const registerDashboardRoutes = (
       appState?: Record<string, unknown>;
       preview?: string | null;
       files?: Record<string, unknown>;
+      version?: number;
     };
+    const isSceneUpdate =
+      payload.elements !== undefined ||
+      payload.appState !== undefined ||
+      payload.files !== undefined;
     const data: Prisma.DrawingUpdateInput = { version: { increment: 1 } };
 
     if (payload.name !== undefined) data.name = payload.name;
@@ -334,11 +339,28 @@ export const registerDashboardRoutes = (
       }
     }
 
+    const updateWhere: Prisma.DrawingWhereInput = { id, userId: req.user.id };
+    if (isSceneUpdate && payload.version !== undefined) {
+      updateWhere.version = payload.version;
+    }
+
     const updateResult = await prisma.drawing.updateMany({
-      where: { id, userId: req.user.id },
+      where: updateWhere,
       data,
     });
     if (updateResult.count === 0) {
+      if (isSceneUpdate && payload.version !== undefined) {
+        const latestDrawing = await prisma.drawing.findFirst({
+          where: { id, userId: req.user.id },
+          select: { version: true },
+        });
+        return res.status(409).json({
+          error: "Conflict",
+          code: "VERSION_CONFLICT",
+          message: "Drawing has changed since this editor state was loaded.",
+          currentVersion: latestDrawing?.version ?? null,
+        });
+      }
       return res.status(404).json({ error: "Drawing not found" });
     }
 

frontend/src/api/index.ts

@@ -340,8 +340,8 @@ export const createDrawing = async (
 };
 
 export const updateDrawing = async (id: string, data: Partial<Drawing>) => {
-  const response = await api.put<{ success: true }>(`/drawings/${id}`, data);
+  const response = await api.put<Drawing>(`/drawings/${id}`, data);
-  return response.data;
+  return deserializeDrawing(response.data);
 };
 
 export const deleteDrawing = async (id: string) => {

frontend/src/components/Layout.test.tsx

@@ -0,0 +1,60 @@
+import { fireEvent, render, screen } from "@testing-library/react";
+import { MemoryRouter } from "react-router-dom";
+import { describe, expect, it, vi } from "vitest";
+
+vi.mock("./Sidebar", () => ({
+  Sidebar: () => <div data-testid="sidebar">sidebar</div>,
+}));
+
+vi.mock("./Logo", () => ({
+  Logo: () => <div data-testid="logo">logo</div>,
+}));
+
+vi.mock("./UploadStatus", () => ({
+  UploadStatus: () => <div data-testid="upload-status">upload-status</div>,
+}));
+
+import { Layout } from "./Layout";
+
+describe("Layout", () => {
+  it("removes active resize listeners on unmount", () => {
+    const addSpy = vi.spyOn(document, "addEventListener");
+    const removeSpy = vi.spyOn(document, "removeEventListener");
+
+    const { unmount } = render(
+      <MemoryRouter>
+        <Layout
+          collections={[]}
+          selectedCollectionId={undefined}
+          onSelectCollection={() => {}}
+          onCreateCollection={() => {}}
+          onEditCollection={() => {}}
+          onDeleteCollection={() => {}}
+        >
+          <div>content</div>
+        </Layout>
+      </MemoryRouter>
+    );
+
+    fireEvent.mouseDown(screen.getByTitle("Drag to resize sidebar"));
+
+    const mouseMoveAdd = addSpy.mock.calls.find(([event]) => event === "mousemove");
+    const mouseUpAdd = addSpy.mock.calls.find(([event]) => event === "mouseup");
+
+    expect(mouseMoveAdd?.[1]).toBeTypeOf("function");
+    expect(mouseUpAdd?.[1]).toBeTypeOf("function");
+
+    unmount();
+
+    expect(
+      removeSpy.mock.calls.some(
+        ([event, handler]) => event === "mousemove" && handler === mouseMoveAdd?.[1]
+      )
+    ).toBe(true);
+    expect(
+      removeSpy.mock.calls.some(
+        ([event, handler]) => event === "mouseup" && handler === mouseUpAdd?.[1]
+      )
+    ).toBe(true);
+  });
+});

frontend/src/context/AuthContext.test.tsx

@@ -0,0 +1,48 @@
+import { render, screen, waitFor } from "@testing-library/react";
+import axios from "axios";
+import { MemoryRouter } from "react-router-dom";
+import { describe, expect, it, vi } from "vitest";
+import { AuthProvider, useAuth } from "./AuthContext";
+
+const Probe = () => {
+  const { loading, authEnabled } = useAuth();
+  return (
+    <div>
+      <span data-testid="loading">{String(loading)}</span>
+      <span data-testid="auth-enabled">{String(authEnabled)}</span>
+    </div>
+  );
+};
+
+describe("AuthProvider", () => {
+  it("defaults to auth-enabled mode if /auth/status fails", async () => {
+    const storage = new Map<string, string>();
+    Object.defineProperty(window, "localStorage", {
+      configurable: true,
+      value: {
+        getItem: (key: string) => storage.get(key) ?? null,
+        setItem: (key: string, value: string) => {
+          storage.set(key, value);
+        },
+        removeItem: (key: string) => {
+          storage.delete(key);
+        },
+      },
+    });
+
+    vi.spyOn(axios, "get").mockRejectedValueOnce(new Error("network down"));
+
+    render(
+      <MemoryRouter>
+        <AuthProvider>
+          <Probe />
+        </AuthProvider>
+      </MemoryRouter>
+    );
+
+    await waitFor(() => {
+      expect(screen.getByTestId("loading").textContent).toBe("false");
+    });
+    expect(screen.getByTestId("auth-enabled").textContent).toBe("true");
+  });
+});

frontend/src/pages/Dashboard.tsx

@@ -11,6 +11,7 @@ import clsx from 'clsx';
 import { ConfirmModal } from '../components/ConfirmModal';
 import { useUpload } from '../context/UploadContext';
 import { DragOverlayPortal, getSelectionBounds, type Point, type SelectionBounds } from './dashboard/shared';
+import { isLatestRequest, mergeUniqueDrawings } from './dashboard/pagination';
 
 const PAGE_SIZE = 24;
 
@@ -92,7 +93,7 @@ export const Dashboard: React.FC = () => {
         }),
         api.getCollections()
       ]);
-      if (requestVersion !== listRequestVersionRef.current) return;
+      if (!isLatestRequest(requestVersion, listRequestVersionRef.current)) return;
       setDrawings(drawingsRes.drawings);
       setTotalCount(drawingsRes.totalCount);
       setCollections(collectionsData);
@@ -100,7 +101,7 @@ export const Dashboard: React.FC = () => {
     } catch (err) {
       console.error('Failed to fetch data:', err);
     } finally {
-      if (requestVersion === listRequestVersionRef.current) {
+      if (isLatestRequest(requestVersion, listRequestVersionRef.current)) {
         setIsLoading(false);
       }
     }
@@ -117,12 +118,8 @@ export const Dashboard: React.FC = () => {
         sortField: sortConfig.field,
         sortDirection: sortConfig.direction,
       });
-      if (requestVersion !== listRequestVersionRef.current) return;
+      if (!isLatestRequest(requestVersion, listRequestVersionRef.current)) return;
-      setDrawings(prev => {
+      setDrawings(prev => mergeUniqueDrawings(prev, drawingsRes.drawings));
-        const seen = new Set(prev.map((d) => d.id));
-        const nextPage = drawingsRes.drawings.filter((d) => !seen.has(d.id));
-        return [...prev, ...nextPage];
-      });
       setTotalCount(drawingsRes.totalCount);
     } catch (err) {
       console.error('Failed to fetch more data:', err);

frontend/src/pages/Editor.tsx

@@ -19,7 +19,9 @@ import {
   getColorFromString,
   getFilesDelta,
   getInitialsFromName,
+  hasRenderableElements,
   haveSameElements,
+  isSuspiciousEmptySnapshot,
 } from './editor/shared';
 import type { ElementVersionInfo } from './editor/shared';
 
@@ -124,7 +126,9 @@ export const Editor: React.FC = () => {
   const latestFilesRef = useRef<any>(null);
   const lastSyncedFilesRef = useRef<Record<string, any>>({});
   const latestAppStateRef = useRef<any>(null);
-  const debouncedSaveRef = useRef<((drawingId: string, elements: readonly any[], appState: any) => void) | null>(null);
+  const debouncedSaveRef = useRef<((drawingId: string, elements: readonly any[], appState: any, files?: Record<string, any>) => void) | null>(null);
+  const currentDrawingVersionRef = useRef<number | null>(null);
+  const lastPersistedElementsRef = useRef<readonly any[]>([]);
 
   const emitFilesDeltaIfNeeded = useCallback(
     (nextFiles: Record<string, any>) => {
@@ -362,7 +366,7 @@ export const Editor: React.FC = () => {
 
         // Persist after file data becomes available so new tabs (tab3) load correctly.
         if (didEmit && id && latestAppStateRef.current && debouncedSaveRef.current) {
-          debouncedSaveRef.current(id, latestElementsRef.current, latestAppStateRef.current);
+          debouncedSaveRef.current(id, latestElementsRef.current, latestAppStateRef.current, latestFilesRef.current || {});
         }
       };
     }
@@ -428,11 +432,11 @@ export const Editor: React.FC = () => {
     scrollToContent: true,
   }), []);
 
-  const saveDataRef = useRef<((drawingId: string, elements: readonly any[], appState: any) => Promise<void>) | null>(null);
+  const saveDataRef = useRef<((drawingId: string, elements: readonly any[], appState: any, files?: Record<string, any>) => Promise<void>) | null>(null);
   const savePreviewRef = useRef<((drawingId: string, elements: readonly any[], appState: any, files: any) => Promise<void>) | null>(null);
   const saveLibraryRef = useRef<((items: any[]) => Promise<void>) | null>(null);
 
-  saveDataRef.current = async (drawingId: string, elements: readonly any[], appState: any) => {
+  saveDataRef.current = async (drawingId: string, elements: readonly any[], appState: any, files?: Record<string, any>) => {
     if (!drawingId) return;
 
     try {
@@ -442,24 +446,38 @@ export const Editor: React.FC = () => {
         gridSize: appState?.gridSize || null,
       };
 
-      const snapshot = latestElementsRef.current ?? elements ?? [];
+      const persistableElements = Array.isArray(elements) ? elements : [];
-      const persistableElements = Array.isArray(snapshot) ? snapshot : [];
+      if (isSuspiciousEmptySnapshot(lastPersistedElementsRef.current, persistableElements)) {
+        console.warn("[Editor] Skipping suspicious empty snapshot save", { drawingId });
+        return;
+      }
+      const persistableFiles = files ?? latestFilesRef.current ?? {};
 
       console.log("[Editor] Saving drawing", {
         drawingId,
         elementCount: persistableElements.length,
-        hasRenderableElements: persistableElements.some((el: any) => !el?.isDeleted),
+        hasRenderableElements: hasRenderableElements(persistableElements),
         appState: persistableAppState,
       });
 
-      await api.updateDrawing(drawingId, {
+      const updated = await api.updateDrawing(drawingId, {
         elements: persistableElements,
         appState: persistableAppState,
-        files: latestFilesRef.current || {},
+        files: persistableFiles,
+        version: currentDrawingVersionRef.current ?? undefined,
       });
+      if (typeof updated.version === "number") {
+        currentDrawingVersionRef.current = updated.version;
+      }
+      lastPersistedElementsRef.current = persistableElements;
 
       console.log("[Editor] Save complete", { drawingId });
     } catch (err) {
+      if (api.isAxiosError(err) && err.response?.status === 409) {
+        console.warn("[Editor] Version conflict while saving drawing", { drawingId });
+        toast.error("Drawing changed in another tab. Refresh to load latest.");
+        return;
+      }
       console.error('Failed to save drawing', err);
       toast.error("Failed to save changes");
     }
@@ -509,9 +527,9 @@ export const Editor: React.FC = () => {
 
 
   const debouncedSave = useCallback(
-    debounce((drawingId, elements, appState) => {
+    debounce((drawingId, elements, appState, files) => {
       if (saveDataRef.current) {
-        saveDataRef.current(drawingId, elements, appState);
+        saveDataRef.current(drawingId, elements, appState, files);
       }
     }, 1000),
     [] // Empty dependency array = Stable across renders
@@ -587,6 +605,8 @@ export const Editor: React.FC = () => {
     latestElementsRef.current = [];
     latestFilesRef.current = {};
     lastSyncedFilesRef.current = {};
+    currentDrawingVersionRef.current = null;
+    lastPersistedElementsRef.current = [];
     excalidrawAPI.current = null;
     setIsReady(false);
     setIsSceneLoading(true);
@@ -614,6 +634,8 @@ export const Editor: React.FC = () => {
         latestElementsRef.current = elements;
         latestFilesRef.current = files;
         lastSyncedFilesRef.current = files;
+        currentDrawingVersionRef.current = typeof data.version === "number" ? data.version : null;
+        lastPersistedElementsRef.current = elements;
 
         elements.forEach((el: any) => {
           recordElementVersion(el);
@@ -657,6 +679,8 @@ export const Editor: React.FC = () => {
         latestElementsRef.current = [];
         latestFilesRef.current = {};
         lastSyncedFilesRef.current = {};
+        currentDrawingVersionRef.current = null;
+        lastPersistedElementsRef.current = [];
         setLoadError(message);
         setInitialData(null);
       } finally {
@@ -678,7 +702,7 @@ export const Editor: React.FC = () => {
           latestElementsRef.current = elements;
           latestFilesRef.current = files;
           if (!id) return;
-          await saveDataRef.current(id, elements, appState);
+          await saveDataRef.current(id, elements, appState, files);
           savePreviewRef.current(id, elements, appState, files);
           toast.success("Saved changes to server");
         }
@@ -729,8 +753,8 @@ export const Editor: React.FC = () => {
 
     latestElementsRef.current = allElements;
 
-    const hasRenderableElements = allElements.some((el: any) => !el?.isDeleted);
+    const hasRenderable = hasRenderableElements(allElements);
-    if (isBootstrappingScene.current && !hasRenderableElements) {
+    if (isBootstrappingScene.current && !hasRenderable) {
       console.log("[Editor] Bootstrapping guard active", {
         drawingId: id,
         elementCount: allElements.length,
@@ -741,19 +765,20 @@ export const Editor: React.FC = () => {
     // Trigger Sync (Throttled)
     broadcastChanges(allElements, currentFiles);
 
+    const filesSnapshot = currentFiles;
+    latestFilesRef.current = filesSnapshot;
+
     // Trigger Fast Save
     console.log("[Editor] Queueing save", {
       drawingId: id,
       elementCount: allElements.length,
-      hasRenderableElements,
+      hasRenderableElements: hasRenderable,
     });
     if (id) {
-      debouncedSave(id, allElements, appState);
+      debouncedSave(id, allElements, appState, filesSnapshot);
     }
 
     // Trigger Slow Preview Gen
-    const filesSnapshot = currentFiles;
-    latestFilesRef.current = filesSnapshot;
     console.log("[Editor] Queueing preview save", {
       drawingId: id,
       fileCount: Object.keys(filesSnapshot).length,
@@ -779,7 +804,7 @@ export const Editor: React.FC = () => {
 
       // Persist after file data becomes available (covers the "tab 3" case).
       if (didEmit && latestAppStateRef.current && debouncedSaveRef.current) {
-        debouncedSaveRef.current(id, latestElementsRef.current, latestAppStateRef.current);
+        debouncedSaveRef.current(id, latestElementsRef.current, latestAppStateRef.current, nextFiles);
       }
     }, 1000);
 
@@ -823,7 +848,7 @@ export const Editor: React.FC = () => {
         latestFilesRef.current = files;
 
         await Promise.all([
-          saveDataRef.current(id, elements, appState),
+          saveDataRef.current(id, elements, appState, files),
           savePreviewRef.current(id, elements, appState, files)
         ]);
         console.log("[Editor] Saved on back navigation", { drawingId: id });

frontend/src/pages/dashboard/pagination.test.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from "vitest";
+import { isLatestRequest, mergeUniqueDrawings } from "./pagination";
+import type { DrawingSummary } from "../../types";
+
+const drawing = (id: string, name = id): DrawingSummary => ({
+  id,
+  name,
+  collectionId: null,
+  preview: null,
+  version: 1,
+  createdAt: Date.now(),
+  updatedAt: Date.now(),
+});
+
+describe("dashboard pagination helpers", () => {
+  it("accepts only latest request version", () => {
+    expect(isLatestRequest(3, 3)).toBe(true);
+    expect(isLatestRequest(2, 3)).toBe(false);
+    expect(isLatestRequest(4, 3)).toBe(false);
+  });
+
+  it("merges pages without duplicating IDs", () => {
+    const existing = [drawing("a"), drawing("b")];
+    const incoming = [drawing("b", "b-new"), drawing("c")];
+
+    const merged = mergeUniqueDrawings(existing, incoming);
+
+    expect(merged.map((d) => d.id)).toEqual(["a", "b", "c"]);
+    expect(merged[1].name).toBe("b");
+  });
+});

frontend/src/pages/dashboard/pagination.ts

@@ -0,0 +1,13 @@
+import type { DrawingSummary } from "../../types";
+
+export const isLatestRequest = (requestVersion: number, currentVersion: number): boolean =>
+  requestVersion === currentVersion;
+
+export const mergeUniqueDrawings = (
+  existing: DrawingSummary[],
+  incoming: DrawingSummary[]
+): DrawingSummary[] => {
+  const seen = new Set(existing.map((d) => d.id));
+  const nextPage = incoming.filter((d) => !seen.has(d.id));
+  return [...existing, ...nextPage];
+};

frontend/src/pages/editor/shared.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+import {
+  hasRenderableElements,
+  isSuspiciousEmptySnapshot,
+} from "./shared";
+
+describe("editor/shared scene guards", () => {
+  it("detects renderable elements", () => {
+    expect(hasRenderableElements([{ id: "a", isDeleted: false }])).toBe(true);
+    expect(
+      hasRenderableElements([
+        { id: "a", isDeleted: true },
+        { id: "b", isDeleted: true },
+      ])
+    ).toBe(false);
+  });
+
+  it("flags empty snapshot after a previously non-empty persisted scene", () => {
+    const previous = [{ id: "a", isDeleted: false }];
+    expect(isSuspiciousEmptySnapshot(previous, [])).toBe(true);
+  });
+
+  it("does not flag empty snapshot for already-empty drawings", () => {
+    expect(isSuspiciousEmptySnapshot([], [])).toBe(false);
+  });
+
+  it("does not flag non-empty snapshots", () => {
+    const previous = [{ id: "a", isDeleted: false }];
+    const next = [{ id: "a", isDeleted: true }];
+    expect(isSuspiciousEmptySnapshot(previous, next)).toBe(false);
+  });
+});

frontend/src/pages/editor/shared.ts

@@ -17,6 +17,21 @@ export const haveSameElements = (a: readonly any[] = [], b: readonly any[] = [])
   return true;
 };
 
+export const hasRenderableElements = (elements: readonly any[] = []): boolean =>
+  elements.some((element: any) => !element?.isDeleted);
+
+/**
+ * Guard against transient empty snapshots (e.g. hydration/reload races) from
+ * overwriting a previously persisted non-empty drawing.
+ */
+export const isSuspiciousEmptySnapshot = (
+  previousPersisted: readonly any[] = [],
+  nextSnapshot: readonly any[] = []
+): boolean => {
+  if (!Array.isArray(nextSnapshot) || nextSnapshot.length > 0) return false;
+  return hasRenderableElements(previousPersisted);
+};
+
 const buildFileSignature = (file: any): string => {
   const mimeType = typeof file?.mimeType === "string" ? file.mimeType : "";
   const id = typeof file?.id === "string" ? file.id : "";