diff --git a/src/components/image-editor/ImageCanvasEditorView.tsx b/src/components/image-editor/ImageCanvasEditorView.tsx
index dc4512b39..1e50147d2 100644
--- a/src/components/image-editor/ImageCanvasEditorView.tsx
+++ b/src/components/image-editor/ImageCanvasEditorView.tsx
@@ -1116,7 +1116,6 @@ export function ImageCanvasEditorView({
     () => ({
       layersRef,
       viewportRef,
-      canvasViewportRef,
       canvasGenerationDialogsRef,
       canvasBackgroundColorRef,
     }),
@@ -1156,6 +1155,7 @@ export function ImageCanvasEditorView({
     layers,
     canvasGenerationDialogs,
     viewport,
+    canvasSize,
     canvasBackgroundColor,
     isViewportInteracting,
     canAccessProtectedData: authUi ? authUi.canAccessProtectedData : true,
diff --git a/src/components/image-editor/useImageCanvasProjectPersistence.test.tsx b/src/components/image-editor/useImageCanvasProjectPersistence.test.tsx
index 8ebb77d8f..1673a2011 100644
--- a/src/components/image-editor/useImageCanvasProjectPersistence.test.tsx
+++ b/src/components/image-editor/useImageCanvasProjectPersistence.test.tsx
@@ -113,12 +113,12 @@ function ProjectPersistenceHarness({
     y: 0,
     scale: 1,
   });
+  const canvasSize = { width: 900, height: 640 };
   const [isViewportInteracting, setIsViewportInteracting] = useState(false);
   const [projectTitle, setProjectTitle] = useState('');
   const [projectRenameValue, setProjectRenameValue] = useState('');
   const layersRef = useRef(layers);
   const viewportRef = useRef(viewport);
-  const canvasViewportRef = useRef<HTMLDivElement | null>(null);
   const canvasGenerationDialogsRef = useRef(generationDialogs);
   const canvasBackgroundColorRef = useRef(canvasBackgroundColor);
   const selectedLayerRef = useRef<string | null>(null);
@@ -147,7 +147,6 @@ function ProjectPersistenceHarness({
     () => ({
       layersRef,
       viewportRef,
-      canvasViewportRef,
       canvasGenerationDialogsRef,
       canvasBackgroundColorRef,
     }),
@@ -173,6 +172,7 @@ function ProjectPersistenceHarness({
     layers,
     canvasGenerationDialogs: generationDialogs,
     viewport,
+    canvasSize,
     canvasBackgroundColor,
     isViewportInteracting,
     canAccessProtectedData,
diff --git a/src/components/image-editor/useImageCanvasProjectPersistence.ts b/src/components/image-editor/useImageCanvasProjectPersistence.ts
index b1617927e..44a447ce4 100644
--- a/src/components/image-editor/useImageCanvasProjectPersistence.ts
+++ b/src/components/image-editor/useImageCanvasProjectPersistence.ts
@@ -22,7 +22,6 @@ import {
   canvasDisplayViewportToViewport,
   type CanvasLayerResourceMetadata,
   DEFAULT_CANVAS_BACKGROUND_COLOR,
-  DEFAULT_CANVAS_SIZE,
   hydrateLayer,
   isInlineEditorMediaSource,
   serializeCanvasLayout,
@@ -40,6 +39,7 @@ import {
   PROJECT_COVER_SNAPSHOT_ASSET_KIND,
   PROJECT_COVER_SNAPSHOT_OBJECT_ASSET_KIND,
   PROJECT_COVER_SNAPSHOT_SIZE,
+  type ProjectCoverSnapshotViewportSize,
 } from './ImageCanvasProjectCoverSnapshotModel';
 
 type ProjectResourceOptions = {
@@ -66,7 +66,6 @@ type CachedEditorProjectSnapshot = {
 type ImageCanvasProjectPersistenceRefs = {
   layersRef: RefObject<CanvasLayer[]>;
   viewportRef: RefObject<CanvasViewport>;
-  canvasViewportRef: RefObject<HTMLDivElement | null>;
   canvasGenerationDialogsRef: RefObject<CanvasGenerationDialogState[]>;
   canvasBackgroundColorRef: RefObject<string>;
 };
@@ -90,6 +89,7 @@ type ImageCanvasProjectPersistenceOptions = {
   layers: CanvasLayer[];
   canvasGenerationDialogs: CanvasGenerationDialogState[];
   viewport: CanvasViewport;
+  canvasSize: ProjectCoverSnapshotViewportSize;
   canvasBackgroundColor: string;
   isViewportInteracting: boolean;
   canAccessProtectedData: boolean;
@@ -116,21 +116,6 @@ function isEditorProjectRevisionConflict(error: unknown) {
   return error instanceof ApiClientError && error.status === 409;
 }
 
-function resolveProjectCoverViewportSize(
-  viewportElement: HTMLDivElement | null,
-) {
-  const width = viewportElement?.clientWidth ?? 0;
-  const height = viewportElement?.clientHeight ?? 0;
-  return {
-    width:
-      width > 0 && Number.isFinite(width) ? width : DEFAULT_CANVAS_SIZE.width,
-    height:
-      height > 0 && Number.isFinite(height)
-        ? height
-        : DEFAULT_CANVAS_SIZE.height,
-  };
-}
-
 const EDITOR_PROJECT_LAYOUT_RETRYABLE_STATUS_CODES = new Set([
   408, 425, 429, 502, 503, 504,
 ]);
@@ -330,6 +315,7 @@ export function useImageCanvasProjectPersistence({
   layers,
   canvasGenerationDialogs,
   viewport,
+  canvasSize,
   canvasBackgroundColor,
   isViewportInteracting,
   canAccessProtectedData,
@@ -354,6 +340,7 @@ export function useImageCanvasProjectPersistence({
   const projectTitleRef = useRef('未命名画布');
   const coverSnapshotSignatureRef = useRef<string | null>(null);
   const coverSnapshotUploadRequestRef = useRef(0);
+  const coverSnapshotViewportSizeRef = useRef(canvasSize);
   const [projectId, setProjectId] = useState<string | null>(null);
   const [isProjectReady, setIsProjectReady] = useState(false);
   const {
@@ -366,6 +353,7 @@ export function useImageCanvasProjectPersistence({
     restoreCanvasGenerationDialogs,
     applyCanvasBackgroundColor,
   } = setters;
+  coverSnapshotViewportSizeRef.current = canvasSize;
 
   const clearPendingProjectLayoutSave = useCallback(() => {
     pendingProjectLayoutSaveRef.current = null;
@@ -478,9 +466,7 @@ export function useImageCanvasProjectPersistence({
       }
       const coverViewport =
         canvasDisplayViewportToViewport(coverDisplayViewport);
-      const viewportSize = resolveProjectCoverViewportSize(
-        refs.canvasViewportRef.current,
-      );
+      const viewportSize = coverSnapshotViewportSizeRef.current;
       const backgroundColor = refs.canvasBackgroundColorRef.current;
       const signature = buildProjectCoverSnapshotSignature({
         layers: coverLayers,