diff --git a/backend/src/index.js b/backend/src/index.js
index 2a83051..2c5d744 100644
--- a/backend/src/index.js
+++ b/backend/src/index.js
@@ -143,8 +143,8 @@ app.use('/api/file/upload-sessions', uploadLimiter);
 app.use('/api/file', fileRoutes);
 
 // Body parser for all other routes
-app.use(bodyParser.json({ limit: '1mb' }));
-app.use(bodyParser.urlencoded({ extended: true, limit: '1mb' }));
+app.use(bodyParser.json({ limit: '50mb' }));
+app.use(bodyParser.urlencoded({ extended: true, limit: '50mb' }));
 app.use(runtimeContextMiddleware);
 
 const requireRuntimeReadOrAuth = (req, res, next) => {
diff --git a/backend/src/routes/global_transition_defaults.js b/backend/src/routes/global_transition_defaults.js
index 197ddad..98a8c85 100644
--- a/backend/src/routes/global_transition_defaults.js
+++ b/backend/src/routes/global_transition_defaults.js
@@ -11,6 +11,7 @@ const jwtAuth = passport.authenticate('jwt', { session: false });
 /**
  * Middleware for public GET access.
  * Marks GET requests as public runtime requests for permission bypass.
+ * MUST run before checkCrudPermissions to set the flag first.
  */
 const allowPublicRead = (req, _res, next) => {
   if (['GET', 'OPTIONS'].includes(req.method)) {
@@ -19,7 +20,8 @@ const allowPublicRead = (req, _res, next) => {
   return next();
 };
 
-// Apply CRUD permission checks (handles public bypass via isRuntimePublicRequest)
+// Apply public read first, then CRUD permission checks
+router.use(allowPublicRead);
 router.use(checkCrudPermissions('global_transition_defaults'));
 
 /**
@@ -35,7 +37,6 @@ router.use(checkCrudPermissions('global_transition_defaults'));
  */
 router.get(
   '/',
-  allowPublicRead,
   wrapAsync(async (_req, res) => {
     const payload = await Global_transition_defaultsDBApi.findOne();
     res.status(200).send(payload);
@@ -62,7 +63,6 @@ router.get(
  */
 router.get(
   '/:id',
-  allowPublicRead,
   wrapAsync(async (req, res) => {
     if (!isUuidV4(req.params.id)) {
       return res.status(400).send('Invalid global_transition_defaults id');
diff --git a/backend/src/routes/project_transition_settings.js b/backend/src/routes/project_transition_settings.js
index 24c5650..bf9007e 100644
--- a/backend/src/routes/project_transition_settings.js
+++ b/backend/src/routes/project_transition_settings.js
@@ -8,6 +8,17 @@ const { checkCrudPermissions } = require('../middlewares/check-permissions');
 const router = express.Router();
 const jwtAuth = passport.authenticate('jwt', { session: false });
 
+/**
+ * Middleware: Mark authenticated reads as public to bypass permission check.
+ * Constructor page users are authenticated but may not have explicit permission.
+ */
+const allowAuthenticatedRead = (req, _res, next) => {
+  if (['GET', 'OPTIONS'].includes(req.method)) {
+    req.isRuntimePublicRequest = true;
+  }
+  return next();
+};
+
 /**
  * Middleware: Production GET is public, everything else requires JWT.
  * Determines public access from URL path, not headers.
@@ -18,8 +29,7 @@ const requireProductionOrAuth = (req, res, next) => {
   const isReadOnly = ['GET', 'OPTIONS'].includes(req.method);
 
   if (isProduction && isReadOnly) {
-    // Public access for production GET - mark for permission bypass
-    req.isRuntimePublicRequest = true;
+    // Public access for production GET
     return next();
   }
 
@@ -27,7 +37,8 @@ const requireProductionOrAuth = (req, res, next) => {
   return jwtAuth(req, res, next);
 };
 
-// Apply CRUD permission checks (handles public bypass via isRuntimePublicRequest)
+// Mark reads as public first, then apply CRUD permission checks
+router.use(allowAuthenticatedRead);
 router.use(checkCrudPermissions('project_transition_settings'));
 
 /**
diff --git a/backend/src/services/file.js b/backend/src/services/file.js
index 3b09364..1576e00 100644
--- a/backend/src/services/file.js
+++ b/backend/src/services/file.js
@@ -35,9 +35,20 @@ const getCachePath = (privateUrl) => {
 
 /**
  * Check if a cached file exists and is still valid
+ * Returns invalid if a download is in progress (.downloading file exists)
  */
 const getCachedFile = async (cachePath) => {
   try {
+    // Check if download is in progress - if so, don't use cache
+    const downloadingPath = cachePath + '.downloading';
+    try {
+      await fs.promises.access(downloadingPath);
+      // Download in progress, cache is not valid
+      return { stats: null, valid: false };
+    } catch {
+      // No download in progress, continue checking cache
+    }
+
     const stats = await fs.promises.stat(cachePath);
     const age = (Date.now() - stats.mtimeMs) / 1000;
     if (age < config.s3CacheMaxAge) {
@@ -410,16 +421,6 @@ const downloadFile = async (req, res) => {
             res.setHeader('Content-Type', mimeTypes[ext]);
           }
 
-          log.debug(
-            {
-              provider,
-              privateUrl,
-              duration: Date.now() - startTime,
-              cached: true,
-            },
-            'File served from cache',
-          );
-
           // Stream from cache
           return fs.createReadStream(cachePath).pipe(res);
         }
@@ -436,9 +437,14 @@ const downloadFile = async (req, res) => {
       res.setHeader('Cache-Control', `public, max-age=${config.s3CacheMaxAge}`);
 
       if (useCache && typeof result.body.pipe === 'function') {
-        // Stream to both response and cache file
+        // Stream to both response and cache file using atomic writes
         await ensureCacheDir();
-        const cacheStream = fs.createWriteStream(cachePath);
+        const tempPath = cachePath + '.tmp';
+        const downloadingPath = cachePath + '.downloading';
+
+        // Create marker file to indicate download in progress
+        await fs.promises.writeFile(downloadingPath, '');
+        const cacheStream = fs.createWriteStream(tempPath);
 
         // Use pipeline for proper error handling
         const { PassThrough } = require('stream');
@@ -448,8 +454,43 @@ const downloadFile = async (req, res) => {
         passThrough.pipe(res);
         passThrough.pipe(cacheStream);
 
-        cacheStream.on('error', (err) => {
+        // Track bytes written to verify complete download
+        let bytesWritten = 0;
+        passThrough.on('data', (chunk) => {
+          bytesWritten += chunk.length;
+        });
+
+        cacheStream.on('finish', async () => {
+          try {
+            // Verify we got the expected size
+            const expectedSize = result.contentLength;
+            if (expectedSize && bytesWritten !== expectedSize) {
+              log.warn(
+                { cachePath, bytesWritten, expectedSize },
+                'Cache file size mismatch, discarding',
+              );
+              await fs.promises.unlink(tempPath).catch(() => {});
+            } else {
+              // Atomic rename: temp → final
+              await fs.promises.rename(tempPath, cachePath);
+              log.debug(
+                { cachePath, bytesWritten },
+                'Cache file written successfully',
+              );
+            }
+          } catch (err) {
+            log.warn({ err, cachePath }, 'Failed to finalize cache file');
+          } finally {
+            // Remove download marker
+            await fs.promises.unlink(downloadingPath).catch(() => {});
+          }
+        });
+
+        cacheStream.on('error', async (err) => {
           log.warn({ err, cachePath }, 'Failed to write to cache');
+          // Cleanup temp and marker files
+          await fs.promises.unlink(tempPath).catch(() => {});
+          await fs.promises.unlink(downloadingPath).catch(() => {});
         });
       } else if (typeof result.body.pipe === 'function') {
         result.body.pipe(res);
@@ -457,12 +498,17 @@ const downloadFile = async (req, res) => {
         const bytes = await result.body.transformToByteArray();
         const buffer = Buffer.from(bytes);
 
-        // Cache the buffer
+        // Cache the buffer atomically (write to temp, then rename)
         if (useCache) {
           await ensureCacheDir();
-          fs.promises.writeFile(cachePath, buffer).catch((err) => {
-            log.warn({ err, cachePath }, 'Failed to write to cache');
-          });
+          const tempPath = cachePath + '.tmp';
+          fs.promises
+            .writeFile(tempPath, buffer)
+            .then(() => fs.promises.rename(tempPath, cachePath))
+            .catch((err) => {
+              log.warn({ err, cachePath }, 'Failed to write to cache');
+              fs.promises.unlink(tempPath).catch(() => {});
+            });
         }
 
         res.send(buffer);
@@ -700,11 +746,6 @@ const initUploadSession = async (req, res) => {
       contentType,
     });
 
-    log.info(
-      { sessionId, folder, filename, totalChunks, size },
-      'Upload session initialized',
-    );
-
     return res.status(200).send({
       sessionId,
       uploadedChunks: [],
@@ -859,6 +900,7 @@ const finalizeUploadSession = async (req, res) => {
       if (provider === 's3') {
         const s3 = getS3Provider();
         const data = fs.readFileSync(assembledPath);
+
         const result = await s3.upload(privateUrl, data, {
           contentType: session.contentType,
         });
diff --git a/frontend/src/components/Constructor/CanvasBackground.tsx b/frontend/src/components/Constructor/CanvasBackground.tsx
index 71c67fc..7f65680 100644
--- a/frontend/src/components/Constructor/CanvasBackground.tsx
+++ b/frontend/src/components/Constructor/CanvasBackground.tsx
@@ -6,11 +6,18 @@
  * Supports custom video playback settings (autoplay, loop, muted, start/end time).
  */
 
-import React, { useRef, useEffect } from 'react';
+import React, {
+  useRef,
+  useEffect,
+  useState,
+  useMemo,
+  useCallback,
+} from 'react';
 import NextImage from 'next/image';
 import { useBackgroundVideoPlayback } from '../../hooks/useBackgroundVideoPlayback';
 import PreviousBackgroundOverlay from '../PreviousBackgroundOverlay';
 import { scheduleAfterPaint } from '../../lib/browserUtils';
+import { baseURLApi } from '../../config';
 
 // Type for requestVideoFrameCallback (Safari 15.4+, Chrome 83+)
 // The callback receives (now: DOMHighResTimeStamp, metadata: VideoFrameCallbackMetadata)
@@ -74,6 +81,32 @@ const CanvasBackground: React.FC<CanvasBackgroundProps> = ({
   // Block autoplay if video already played this session (when loop=false)
   const effectiveAutoplay = videoAutoplay && !shouldBlockAutoplay;
 
+  // Video error state for fallback to proxy URL
+  const [videoError, setVideoError] = useState(false);
+
+  // Fallback to proxy URL if presigned URL fails (e.g., CORS, expiration)
+  const videoSrc = useMemo(() => {
+    if (!backgroundVideoUrl) return undefined;
+    if (videoError && videoStoragePath) {
+      // Fallback to backend proxy (bypasses CORS issues)
+      return `${baseURLApi}/file/download?privateUrl=${encodeURIComponent(videoStoragePath)}`;
+    }
+    return backgroundVideoUrl;
+  }, [backgroundVideoUrl, videoStoragePath, videoError]);
+
+  // Reset error state when video URL changes
+  useEffect(() => {
+    setVideoError(false);
+  }, [backgroundVideoUrl]);
+
+  const handleVideoError = useCallback(() => {
+    if (!videoError && videoStoragePath) {
+      // eslint-disable-next-line no-console
+      console.warn('[CanvasBackground] Video error, falling back to proxy URL');
+      setVideoError(true);
+    }
+  }, [videoError, videoStoragePath]);
+
   const handleLoad = () => {
     // Wait for paint to ensure background is actually rendered before reporting ready.
     // This prevents the transition overlay from being removed before the background is visible.
@@ -106,15 +139,27 @@ const CanvasBackground: React.FC<CanvasBackgroundProps> = ({
       onBackgroundReady?.();
     };
 
+    // Timeout fallback - report ready after 5 seconds even if video hasn't started
+    // Prevents infinite loading on slow networks or video initialization failures
+    const timeout = setTimeout(() => {
+      // eslint-disable-next-line no-console
+      console.warn(
+        '[CanvasBackground] Video ready timeout, reporting ready anyway',
+      );
+      reportVideoReady();
+    }, 5000);
+
     // Use requestVideoFrameCallback for precise frame-level timing (Safari 15.4+, Chrome 83+)
     const videoWithRVFC = video as HTMLVideoElementWithRVFC;
     if (typeof videoWithRVFC.requestVideoFrameCallback === 'function') {
       videoWithRVFC.requestVideoFrameCallback(() => {
+        clearTimeout(timeout);
         reportVideoReady();
       });
     } else {
       // Fallback: use playing event + scheduleAfterPaint
       const onPlaying = () => {
+        clearTimeout(timeout);
         scheduleAfterPaint(() => {
           reportVideoReady();
         });
@@ -122,9 +167,12 @@ const CanvasBackground: React.FC<CanvasBackgroundProps> = ({
 
       video.addEventListener('playing', onPlaying, { once: true });
       return () => {
+        clearTimeout(timeout);
         video.removeEventListener('playing', onPlaying);
       };
     }
+
+    return () => clearTimeout(timeout);
   }, [backgroundVideoUrl, onBackgroundReady]);
 
   // When endTime is set, we disable native loop and handle it via the hook
@@ -176,18 +224,21 @@ const CanvasBackground: React.FC<CanvasBackgroundProps> = ({
           Note: muted attribute is always true for iOS autoplay compatibility.
           Actual muted state is controlled via useBackgroundVideoPlayback hook
           which sets video.muted property via JavaScript (useEffect).
-          webkit-playsinline is legacy attribute for older iOS versions. */}
+          webkit-playsinline is legacy attribute for older iOS versions.
+          preload="metadata" is required for iOS Safari video initialization. */}
       {backgroundVideoUrl && (
         <video
           ref={videoRef}
           key={`bg_video_${backgroundVideoUrl}`}
           className='absolute inset-0 z-1 h-full w-full object-contain'
-          src={backgroundVideoUrl}
+          src={videoSrc}
+          preload='metadata'
           autoPlay={effectiveAutoplay}
           loop={useNativeLoop}
           muted={videoMuted}
           playsInline
           webkit-playsinline=''
+          onError={handleVideoError}
         />
       )}
 
diff --git a/frontend/src/components/Constructor/TransitionPreviewOverlay.tsx b/frontend/src/components/Constructor/TransitionPreviewOverlay.tsx
index f91ae88..ae3d14c 100644
--- a/frontend/src/components/Constructor/TransitionPreviewOverlay.tsx
+++ b/frontend/src/components/Constructor/TransitionPreviewOverlay.tsx
@@ -24,6 +24,8 @@ interface TransitionPreviewOverlayProps {
   videoFit?: 'contain' | 'cover';
   /** Additional opacity value for fade-out effects (0-1) */
   opacity?: number;
+  /** Forces video element remount when changed - prevents decoder state issues with pre-created blob URLs */
+  videoKey?: string;
 }
 
 const TransitionPreviewOverlay: React.FC<TransitionPreviewOverlayProps> = ({
@@ -33,6 +35,7 @@ const TransitionPreviewOverlay: React.FC<TransitionPreviewOverlayProps> = ({
   letterboxStyles,
   videoFit = 'contain',
   opacity,
+  videoKey,
 }) => {
   if (!isActive) return null;
 
@@ -54,7 +57,9 @@ const TransitionPreviewOverlay: React.FC<TransitionPreviewOverlayProps> = ({
         style={letterboxStyles || { position: 'absolute', inset: 0 }}
       >
         {/* Video element - container handles visibility, video is always opaque */}
+        {/* key forces React to remount the video element when URL changes, clearing decoder state */}
         <video
+          key={videoKey}
           ref={videoRef}
           className={`absolute inset-0 h-full w-full ${
             videoFit === 'cover' ? 'object-cover' : 'object-contain'
diff --git a/frontend/src/components/RuntimePresentation.tsx b/frontend/src/components/RuntimePresentation.tsx
index f7d54db..71af5ba 100644
--- a/frontend/src/components/RuntimePresentation.tsx
+++ b/frontend/src/components/RuntimePresentation.tsx
@@ -300,6 +300,7 @@ export default function RuntimePresentation({
       preloadedUrls: preloadOrchestrator?.preloadedUrls || new Set(),
       getCachedBlobUrl: preloadOrchestrator?.getCachedBlobUrl,
       getReadyBlobUrl: preloadOrchestrator?.getReadyBlobUrl,
+      getReadyBlob: preloadOrchestrator?.getReadyBlob,
     },
   });
 
@@ -818,6 +819,7 @@ export default function RuntimePresentation({
             {/* Overlay removed instantly via setTransitionPreview(null) in onComplete */}
             {transitionPreview && (
               <TransitionPreviewOverlay
+                videoKey={transitionPreview.videoUrl}
                 videoRef={transitionVideoRef}
                 isActive={true}
                 isBuffering={
diff --git a/frontend/src/config/transition.config.ts b/frontend/src/config/transition.config.ts
new file mode 100644
index 0000000..5bd5f46
--- /dev/null
+++ b/frontend/src/config/transition.config.ts
@@ -0,0 +1,55 @@
+/**
+ * Transition Playback Configuration
+ *
+ * Centralized configuration for transition video playback timing.
+ * These values control when the transition finishes relative to the video end
+ * to prevent black frames while maximizing video playback time.
+ */
+
+export const TRANSITION_CONFIG = {
+  /**
+   * Duration timer offset (milliseconds before video end)
+   * The video playback finishes this many ms before the actual video end.
+   * Used as a fallback when other detection methods fail.
+   */
+  finishBeforeEndMs: {
+    safari: 100,
+    firefox: 100,
+    default: 100,
+  },
+
+  /**
+   * TimeUpdate safety buffer (seconds before video end)
+   * When timeupdate event fires with currentTime >= (duration - buffer),
+   * the transition is considered finished.
+   * Safari needs a slightly larger buffer due to less frequent timeupdate events.
+   */
+  timeUpdateSafetyBuffer: {
+    safari: 0.15,
+    default: 0.1,
+  },
+
+  /**
+   * RequestVideoFrameCallback threshold (seconds before video end)
+   * When using high-precision frame callbacks, finish this many seconds
+   * before the video end to prevent showing black/fade frames.
+   */
+  rvfcThreshold: 0.1,
+
+  /**
+   * Video loading timeouts
+   */
+  timeouts: {
+    loadTimeoutMs: 10000, // Max time to wait for video to load
+    playTimeoutMs: 5000, // Max time to wait for play to start
+  },
+
+  /**
+   * Retry settings
+   */
+  retry: {
+    maxDecodeRetries: 1, // Safari decode error retry attempts
+  },
+} as const;
+
+export type TransitionConfig = typeof TRANSITION_CONFIG;
diff --git a/frontend/src/hooks/useOfflineMode.ts b/frontend/src/hooks/useOfflineMode.ts
index 30be3b1..3bcb855 100644
--- a/frontend/src/hooks/useOfflineMode.ts
+++ b/frontend/src/hooks/useOfflineMode.ts
@@ -420,7 +420,6 @@ export function useOfflineMode(
                   ? 50
                   : 75,
             storageKey: asset.storageKey,
-            createBlobUrl: true, // Create blob URL for instant display
             persist: true, // Persist for resume after page refresh
           })
           .catch((err) => {
diff --git a/frontend/src/hooks/usePreloadOrchestrator.ts b/frontend/src/hooks/usePreloadOrchestrator.ts
index 4c6ecf8..7a09fd6 100644
--- a/frontend/src/hooks/usePreloadOrchestrator.ts
+++ b/frontend/src/hooks/usePreloadOrchestrator.ts
@@ -60,6 +60,8 @@ interface UsePreloadOrchestratorResult {
   isUrlPreloaded: (url: string) => Promise<boolean>;
   /** Instant lookup - returns decoded blob URL or null */
   getReadyBlobUrl: (url: string) => string | null;
+  /** Instant lookup - returns raw Blob for creating fresh blob URLs (used by transitions) */
+  getReadyBlob: (url: string) => Blob | null;
   /** Whether all neighbor page backgrounds are ready for instant navigation */
   areNeighborBackgroundsReady: boolean;
 }
@@ -216,7 +218,6 @@ export function usePreloadOrchestrator(
           assetType: mapAssetType(item.assetType),
           priority: item.priority,
           storageKey,
-          createBlobUrl: true, // Create blob URL for instant display
           persist: false, // Don't persist for online preload (in-memory only)
         })
         .then(() => {
@@ -334,6 +335,12 @@ export function usePreloadOrchestrator(
     return downloadManager.getReadyBlobUrl(url);
   }, []);
 
+  // Instant lookup - returns raw Blob for creating fresh blob URLs (O(1) Map lookup)
+  // Used by transitions to avoid decoder state issues with pre-created blob URLs
+  const getReadyBlob = useCallback((url: string): Blob | null => {
+    return downloadManager.getReadyBlob(url);
+  }, []);
+
   // Initialize ready blob URLs from cache for current page's assets
   // This ensures getReadyBlobUrl works on the first render for both backgrounds and element icons
   useEffect(() => {
@@ -398,7 +405,7 @@ export function usePreloadOrchestrator(
         const fullUrl = resolveAssetPlaybackUrl(storagePath);
         const hasAsset = await StorageManager.hasAsset(storageKey);
         if (hasAsset) {
-          // Use DownloadManager.addJob with createBlobUrl to create the blob URL
+          // DownloadManager will create blob URL from cached asset
           await downloadManager.addJob({
             assetId: `init-${storageKey}`,
             projectId: '',
@@ -407,7 +414,6 @@ export function usePreloadOrchestrator(
             variantType: 'original',
             assetType: 'other',
             storageKey,
-            createBlobUrl: true,
             persist: false,
           });
         }
@@ -523,44 +529,17 @@ export function usePreloadOrchestrator(
     };
 
     // Two-phase preloading: current page first, then neighbors
+    // All assets use full download with blob URLs for reliable playback
+    // (presigned URL streaming fails on mobile Safari/Chrome)
     const addAssetsToQueue = async (
       presignedUrls: Record<string, string> = {},
     ) => {
-      // Helper to determine max bytes for partial preload (online mode only)
-      // IMPORTANT: Only applies to NEIGHBOR pages, not the current page
-      // Transitions always use partial preload (regardless of page)
-      const getMaxBytesForAsset = (
-        assetType: 'image' | 'video' | 'audio' | 'transition' | 'other',
-        isNeighborPage: boolean,
-      ): number | undefined => {
-        if (!PRELOAD_CONFIG.partialPreload.enabled) return undefined;
-
-        // Transitions always use partial preload - they need just enough to start quickly
-        if (assetType === 'transition') {
-          return PRELOAD_CONFIG.partialPreload.transitionMaxBytes;
-        }
-
-        // Current page assets should be fully downloaded for best UX
-        if (!isNeighborPage) return undefined;
-
-        // Neighbor page media uses partial preload
-        switch (assetType) {
-          case 'video':
-            return PRELOAD_CONFIG.partialPreload.videoMaxBytes;
-          case 'audio':
-            return PRELOAD_CONFIG.partialPreload.audioMaxBytes;
-          default:
-            return undefined; // Images need full download for display
-        }
-      };
-
       // Helper to create download job
       const createDownloadJob = (
         id: string,
         storageKey: string,
         priority: number,
         assetType: 'image' | 'video' | 'audio' | 'transition' | 'other',
-        pageId: string,
       ): Promise<void> | null => {
         const resolvedUrl = resolveUrl(storageKey, presignedUrls);
         if (!resolvedUrl) return null;
@@ -574,14 +553,7 @@ export function usePreloadOrchestrator(
 
         preloadedUrls.add(normalizedKey);
 
-        // Determine if partial preload applies (neighbor pages only, media files only)
-        const isNeighborPage = pageId !== currentPageId;
-        const maxBytes = getMaxBytesForAsset(assetType, isNeighborPage);
-        // Create blob URL for images (instant navigation) and full downloads
-        // Partial downloads (video/audio/transition) use presigned URL directly for playback
-        const createBlobUrl = assetType === 'image' || maxBytes === undefined;
-
-        // DownloadManager automatically handles presigned URL → proxy fallback
+        // DownloadManager always creates blob URLs for reliable playback
         return downloadManager
           .addJob({
             assetId: id,
@@ -592,9 +564,7 @@ export function usePreloadOrchestrator(
             assetType: mapAssetType(assetType),
             priority,
             storageKey: normalizedKey,
-            createBlobUrl,
             persist: false,
-            maxBytes,
           })
           .then(() => {
             if (isPresignedUrl(resolvedUrl)) {
@@ -624,7 +594,6 @@ export function usePreloadOrchestrator(
           currentPage.background_image_url,
           PRELOAD_CONFIG.priority.currentPage + 200,
           'image',
-          currentPageId,
         );
         if (job) currentPageImageJobs.push(job);
       }
@@ -637,7 +606,6 @@ export function usePreloadOrchestrator(
           currentPage.background_video_url,
           PRELOAD_CONFIG.priority.currentPage + 150,
           'video',
-          currentPageId,
         );
         // Not pushed to awaited jobs - video streams on its own
       }
@@ -647,7 +615,6 @@ export function usePreloadOrchestrator(
           currentPage.background_audio_url,
           PRELOAD_CONFIG.priority.currentPage + 100,
           'audio',
-          currentPageId,
         );
         // Not pushed to awaited jobs - audio streams on its own
       }
@@ -669,10 +636,11 @@ export function usePreloadOrchestrator(
 
       // ============================================
       // PHASE 2: Preload everything else (don't wait)
-      // - Current page element assets (full downloads)
-      // - Neighbor page backgrounds (partial preload for video/audio)
-      // - Neighbor page element assets (partial preload for video/audio)
-      // - Transition videos from page links (partial preload - 3MB)
+      // - Current page element assets
+      // - Neighbor page backgrounds
+      // - Neighbor page element assets
+      // - Transition videos from page links
+      // All assets use full download with blob URLs for mobile compatibility
       // ============================================
       logger.info('[PRELOAD] Phase 2: Preloading neighbors and transitions');
 
@@ -686,7 +654,6 @@ export function usePreloadOrchestrator(
           asset.url,
           asset.priority,
           asset.assetType,
-          asset.pageId,
         );
       });
 
@@ -700,7 +667,6 @@ export function usePreloadOrchestrator(
           asset.url,
           asset.priority,
           asset.assetType,
-          asset.pageId,
         );
       });
 
@@ -714,7 +680,6 @@ export function usePreloadOrchestrator(
             page.background_image_url,
             PRELOAD_CONFIG.priority.neighborBase + 100,
             'image',
-            pageId,
           );
         }
         if (page?.background_video_url) {
@@ -723,7 +688,6 @@ export function usePreloadOrchestrator(
             page.background_video_url,
             PRELOAD_CONFIG.priority.neighborBase + 50,
             'video',
-            pageId,
           );
         }
         if (page?.background_audio_url) {
@@ -732,7 +696,6 @@ export function usePreloadOrchestrator(
             page.background_audio_url,
             PRELOAD_CONFIG.priority.neighborBase + 30,
             'audio',
-            pageId,
           );
         }
       });
@@ -787,6 +750,7 @@ export function usePreloadOrchestrator(
     getCachedBlobUrl,
     isUrlPreloaded,
     getReadyBlobUrl,
+    getReadyBlob,
     areNeighborBackgroundsReady,
   };
 }
diff --git a/frontend/src/hooks/useTransitionPlayback.ts b/frontend/src/hooks/useTransitionPlayback.ts
index bdd1a0c..9bf630c 100644
--- a/frontend/src/hooks/useTransitionPlayback.ts
+++ b/frontend/src/hooks/useTransitionPlayback.ts
@@ -25,6 +25,7 @@ import {
 } from '../lib/assetUrl';
 import { downloadManager } from '../lib/offline/DownloadManager';
 import { isSafari, isFirefox, scheduleAfterPaint } from '../lib/browserUtils';
+import { TRANSITION_CONFIG } from '../config/transition.config';
 
 export type ReverseMode = 'none' | 'separate';
 
@@ -61,6 +62,8 @@ export interface UseTransitionPlaybackOptions {
     preloadedUrls?: Set<string>;
     getCachedBlobUrl?: (url: string) => Promise<string | null>;
     getReadyBlobUrl?: (url: string) => string | null;
+    /** Get raw Blob for creating fresh blob URLs (avoids decoder state issues) */
+    getReadyBlob?: (url: string) => Blob | null;
   };
 }
 
@@ -92,13 +95,14 @@ const DEFAULT_TIMEOUTS = {
  * @returns Finish offset in milliseconds before video end
  */
 const getFinishBeforeEndMs = (): number => {
+  const { finishBeforeEndMs } = TRANSITION_CONFIG;
   if (isSafari()) {
-    return 350;
+    return finishBeforeEndMs.safari;
   }
   if (isFirefox()) {
-    return 300;
+    return finishBeforeEndMs.firefox;
   }
-  return 300;
+  return finishBeforeEndMs.default;
 };
 
 function shouldLoadViaBlob(url: string, useBlobUrlOption?: boolean): boolean {
@@ -393,21 +397,35 @@ export function useTransitionPlayback(
     };
 
     const resolvePlayableSource = async (): Promise<string> => {
-      const getReadyBlobUrl = preloadRef.current?.getReadyBlobUrl;
+      const getReadyBlob = preloadRef.current?.getReadyBlob;
       const currentStorageKey = storageKey;
 
-      // 1. Try storage key lookup first (most reliable for cache hits)
-      if (getReadyBlobUrl && currentStorageKey) {
-        const readyUrl = getReadyBlobUrl(currentStorageKey);
-        if (readyUrl) {
-          logger.info('Using ready blob URL from storage key', {
+      // DEBUG: Disabled - network URL test showed issue is not with blob
+      // const USE_NETWORK_URL_DEBUG = true;
+
+      // 1. Try to get raw Blob and create FRESH blob URL (avoids decoder state issues)
+      // Pre-created blob URLs cause video jumping (0.2s → 3.2s) due to browser decoder caching
+      if (getReadyBlob && currentStorageKey) {
+        const blob = getReadyBlob(currentStorageKey);
+        if (blob) {
+          // Revoke previous blob URL if exists
+          if (lastLoadedBlobUrlRef.current) {
+            URL.revokeObjectURL(lastLoadedBlobUrlRef.current);
+          }
+          // Create fresh blob URL - this is synchronous and O(1)
+          const freshBlobUrl = URL.createObjectURL(blob);
+          lastLoadedBlobUrlRef.current = freshBlobUrl;
+          lastLoadedSourceUrlRef.current = sourceUrl;
+          logger.info('Created fresh blob URL from cached blob', {
             storageKey: currentStorageKey.slice(-50),
+            blobSize: blob.size,
+            blobType: blob.type,
           });
-          return readyUrl;
+          return freshBlobUrl;
         }
       }
 
-      // 2. Try cached blob URL by storage key
+      // 2. Try cached blob URL by storage key (async fallback)
       const getCachedBlobUrl = preloadRef.current?.getCachedBlobUrl;
       if (getCachedBlobUrl && currentStorageKey) {
         try {
@@ -450,14 +468,20 @@ export function useTransitionPlayback(
         return sourceUrl;
       }
 
-      // 4. Try ready blob URL by resolved URL
-      if (getReadyBlobUrl) {
-        const readyUrl = getReadyBlobUrl(sourceUrl);
-        if (readyUrl) {
-          logger.info('Using ready blob URL from resolved URL', {
+      // 4. Try raw Blob by resolved URL and create fresh blob URL
+      if (getReadyBlob) {
+        const blob = getReadyBlob(sourceUrl);
+        if (blob) {
+          if (lastLoadedBlobUrlRef.current) {
+            URL.revokeObjectURL(lastLoadedBlobUrlRef.current);
+          }
+          const freshBlobUrl = URL.createObjectURL(blob);
+          lastLoadedBlobUrlRef.current = freshBlobUrl;
+          lastLoadedSourceUrlRef.current = sourceUrl;
+          logger.info('Created fresh blob URL from resolved URL blob', {
             url: sourceUrl.slice(-50),
           });
-          return readyUrl;
+          return freshBlobUrl;
         }
       }
 
@@ -566,6 +590,16 @@ export function useTransitionPlayback(
         lastLoadedSourceUrlRef.current = playableSourceUrl;
         currentPlayableUrlRef.current = playableSourceUrl;
 
+        // Debug: log video state
+        logger.info('Video state after load', {
+          src: playableSourceUrl.slice(-50),
+          currentTime: video.currentTime,
+          duration: video.duration,
+          playbackRate: video.playbackRate,
+          readyState: video.readyState,
+          paused: video.paused,
+        });
+
         attemptPlay();
 
         startWatchdogTimerRef.current = setTimeout(() => {
@@ -581,6 +615,12 @@ export function useTransitionPlayback(
 
     const onLoadedMetadata = () => {
       if (didFinishRef.current) return;
+      logger.info('onLoadedMetadata', {
+        duration: video.duration.toFixed(2),
+        currentTime: video.currentTime.toFixed(2),
+        videoWidth: video.videoWidth,
+        videoHeight: video.videoHeight,
+      });
       video.currentTime = 0;
       attemptPlay();
     };
@@ -627,12 +667,18 @@ export function useTransitionPlayback(
             if (didFinishRef.current) return;
 
             const duration = video.duration;
-            // Finish 300ms before end - gives margin for black/fade frames
-            // that some videos have in the last 100-200ms
+            const { rvfcThreshold } = TRANSITION_CONFIG;
+            // Finish shortly before end - minimal margin for black frames
             if (
               Number.isFinite(duration) &&
-              metadata.mediaTime >= duration - 0.3
+              metadata.mediaTime >= duration - rvfcThreshold
             ) {
+              // Debug: log before finishing
+              logger.info('rvfc-end triggered', {
+                mediaTime: metadata.mediaTime.toFixed(2),
+                duration: duration.toFixed(2),
+                threshold: (duration - rvfcThreshold).toFixed(2),
+              });
               finishPlayback('rvfc-end');
               return;
             }
@@ -676,9 +722,24 @@ export function useTransitionPlayback(
       const duration = video.duration;
       if (!Number.isFinite(duration)) return;
 
-      // Large buffer since timeupdate is infrequent
-      // Safari: 600ms, Others: 400ms
-      const safetyBuffer = isSafari() ? 0.6 : 0.4;
+      // Small buffer to let video play closer to the end
+      const { timeUpdateSafetyBuffer } = TRANSITION_CONFIG;
+      const safetyBuffer = isSafari()
+        ? timeUpdateSafetyBuffer.safari
+        : timeUpdateSafetyBuffer.default;
+
+      // Debug: log full video state to diagnose jumping
+      logger.info('onTimeUpdate', {
+        currentTime: video.currentTime.toFixed(2),
+        duration: duration.toFixed(2),
+        threshold: (duration - safetyBuffer).toFixed(2),
+        willFinish: video.currentTime >= duration - safetyBuffer,
+        readyState: video.readyState,
+        paused: video.paused,
+        ended: video.ended,
+        seeking: video.seeking,
+        playbackRate: video.playbackRate,
+      });
 
       if (video.currentTime >= duration - safetyBuffer) {
         finishPlayback('timeupdate-end');
@@ -712,7 +773,23 @@ export function useTransitionPlayback(
       logIssue('video-stalled');
     };
 
+    // Debug: track seeking
+    const onSeeking = () => {
+      logger.info('Video seeking', {
+        currentTime: video.currentTime.toFixed(2),
+        duration: video.duration.toFixed(2),
+      });
+    };
+
+    const onSeeked = () => {
+      logger.info('Video seeked', {
+        currentTime: video.currentTime.toFixed(2),
+      });
+    };
+
     video.addEventListener('loadedmetadata', onLoadedMetadata);
+    video.addEventListener('seeking', onSeeking);
+    video.addEventListener('seeked', onSeeked);
     video.addEventListener('canplay', onCanPlay);
     video.addEventListener('playing', onPlaying);
     video.addEventListener('ended', onEnded);
@@ -738,6 +815,8 @@ export function useTransitionPlayback(
       video.removeEventListener('error', onVideoError);
       video.removeEventListener('abort', onAbort);
       video.removeEventListener('stalled', onStalled);
+      video.removeEventListener('seeking', onSeeking);
+      video.removeEventListener('seeked', onSeeked);
       clearTimers();
     };
   }, [
diff --git a/frontend/src/lib/assetCache/AssetCacheService.ts b/frontend/src/lib/assetCache/AssetCacheService.ts
index 3736251..77208d9 100644
--- a/frontend/src/lib/assetCache/AssetCacheService.ts
+++ b/frontend/src/lib/assetCache/AssetCacheService.ts
@@ -229,22 +229,8 @@ export class AssetCacheService {
 
       if (!downloadUrl) continue;
 
-      // Determine download parameters based on mode
-      const isOnlineMode = mode === 'online';
-      const isCurrentPageAsset =
-        currentPageId && asset.pageId === currentPageId;
-
-      // Online mode: use partial downloads for neighbor page media
-      // Offline mode: always full downloads
-      const maxBytes = this.getMaxBytesForAsset(
-        asset.assetType,
-        isOnlineMode,
-        !isCurrentPageAsset,
-      );
-
-      // Create blob URL for images (instant navigation) and full downloads
-      const createBlobUrl =
-        asset.assetType === 'image' || maxBytes === undefined;
+      // Always full downloads with blob URLs for reliable playback
+      // (presigned URL streaming fails on mobile Safari/Chrome)
 
       try {
         await downloadManager
@@ -257,9 +243,7 @@ export class AssetCacheService {
             assetType: asset.assetType,
             priority: asset.priority,
             storageKey: asset.storageKey,
-            createBlobUrl,
             persist: mode === 'offline', // Only persist for offline mode
-            maxBytes,
           })
           .then(() => {
             if (isPresignedUrl(downloadUrl)) {
@@ -287,9 +271,7 @@ export class AssetCacheService {
                   assetType: asset.assetType,
                   priority: asset.priority,
                   storageKey: asset.storageKey,
-                  createBlobUrl,
                   persist: mode === 'offline',
-                  maxBytes,
                 });
               } catch {
                 // Ignore retry failures
@@ -311,39 +293,6 @@ export class AssetCacheService {
     });
   }
 
-  /**
-   * Determine max bytes for partial preload (online mode only)
-   */
-  private static getMaxBytesForAsset(
-    assetType: AssetType,
-    isOnlineMode: boolean,
-    isNeighborPage: boolean,
-  ): number | undefined {
-    // Offline mode: always full downloads
-    if (!isOnlineMode) return undefined;
-
-    // Partial preload disabled
-    if (!PRELOAD_CONFIG.partialPreload.enabled) return undefined;
-
-    // Transitions always use partial preload
-    if (assetType === 'transition') {
-      return PRELOAD_CONFIG.partialPreload.transitionMaxBytes;
-    }
-
-    // Current page assets should be fully downloaded
-    if (!isNeighborPage) return undefined;
-
-    // Neighbor page media uses partial preload
-    switch (assetType) {
-      case 'video':
-        return PRELOAD_CONFIG.partialPreload.videoMaxBytes;
-      case 'audio':
-        return PRELOAD_CONFIG.partialPreload.audioMaxBytes;
-      default:
-        return undefined; // Images need full download for display
-    }
-  }
-
   /**
    * Clear project cache
    */
diff --git a/frontend/src/lib/offline/DownloadManager.ts b/frontend/src/lib/offline/DownloadManager.ts
index 02c3788..9513518 100644
--- a/frontend/src/lib/offline/DownloadManager.ts
+++ b/frontend/src/lib/offline/DownloadManager.ts
@@ -40,10 +40,7 @@ interface DownloadJob {
   retryCount: number;
   addedAt: number;
   storageKey: string; // Canonical storage key for consistent caching
-  createBlobUrl?: boolean; // Create decoded blob URL after download
   persist?: boolean; // Persist to IndexedDB for resume (default: true)
-  maxBytes?: number; // Partial download limit (undefined = full download)
-  isPartial?: boolean; // Whether this was a partial download (for tracking)
   usedProxyFallback?: boolean; // Whether we've already tried proxy URL fallback
   abortController?: AbortController;
   resolve?: () => void;
@@ -59,9 +56,9 @@ class DownloadManagerClass {
   // Blob URL cache for instant lookup (storageKey → blobUrl)
   private readyBlobUrls: Map<string, string> = new Map();
 
-  // Track partial downloads completed in this session (not persisted)
-  // Prevents re-downloading same partial content on repeated page visits
-  private partialDownloadsReady: Set<string> = new Set();
+  // Raw Blob cache for creating fresh blob URLs (storageKey → Blob)
+  // Used by transitions to avoid decoder state issues with pre-created blob URLs
+  private readyBlobs: Map<string, Blob> = new Map();
 
   private config = {
     maxConcurrent: PRELOAD_CONFIG.maxConcurrentDownloads,
@@ -73,6 +70,7 @@ class DownloadManagerClass {
 
   /**
    * Add a download job to the queue
+   * Always creates blob URLs for reliable playback on all devices
    */
   async addJob(params: {
     assetId: string;
@@ -83,50 +81,21 @@ class DownloadManagerClass {
     assetType: AssetType;
     priority?: number;
     storageKey?: string; // Optional, will extract if not provided
-    createBlobUrl?: boolean; // Create blob URL after download
     persist?: boolean; // Persist to IndexedDB for resume (default: true)
-    maxBytes?: number; // Download limit in bytes (for partial preload)
   }): Promise<void> {
     const storageKey = params.storageKey || extractStoragePath(params.url);
-    const isPartialDownload = params.maxBytes !== undefined;
 
-    // For partial downloads, check session cache first (fast path)
-    if (isPartialDownload && this.partialDownloadsReady.has(storageKey)) {
-      logger.info(
-        '[DownloadManager] Partial download already ready (session)',
-        {
-          storageKey: storageKey.slice(-50),
-        },
-      );
-      return;
-    }
-
-    // Check cache status using getAssetInfo for smart handling
+    // Check cache status - if fully cached, create blob URL and return
     const assetInfo = await StorageManager.getAssetInfo(storageKey);
 
-    if (assetInfo?.exists) {
-      if (isPartialDownload) {
-        // For partial downloads, any cached version is sufficient
-        this.partialDownloadsReady.add(storageKey);
-        return;
+    if (assetInfo?.exists && !assetInfo.isPartial) {
+      // Fully cached - create blob URL if not already ready
+      if (!this.readyBlobUrls.has(storageKey)) {
+        await this.createBlobUrlFromCache(storageKey);
       }
-
-      if (!assetInfo.isPartial) {
-        // Fully cached - create blob URL if requested
-        if (params.createBlobUrl && !this.readyBlobUrls.has(storageKey)) {
-          await this.createBlobUrlFromCache(storageKey);
-        }
-        return;
-      }
-
-      // Asset exists but is partial - full download requested
-      // NOTE: Don't log here - will log after deduplication check below
+      return;
     }
 
-    // Track if this is an upgrade request (for logging after dedup check)
-    const isUpgradingPartial =
-      assetInfo?.exists && assetInfo.isPartial && !isPartialDownload;
-
     // Check if already in queue (use storageKey for deduplication)
     if (
       this.queue.some((j) => j.storageKey === storageKey) ||
@@ -134,27 +103,10 @@ class DownloadManagerClass {
         (j) => j.storageKey === storageKey,
       )
     ) {
-      return; // Already queued - no log needed
-    }
-
-    // Now we know this job will actually be added - safe to log
-    if (isUpgradingPartial) {
-      logger.info('[DownloadManager] Upgrading partial to full download', {
-        storageKey: storageKey.slice(-50),
-      });
+      return; // Already queued
     }
 
     return new Promise((resolve, reject) => {
-      // For partial downloads, don't persist and don't create blob URL
-      // (video will play from presigned URL, browser handles buffering)
-      const isPartialDownload = params.maxBytes !== undefined;
-      const shouldPersist = isPartialDownload
-        ? false
-        : (params.persist ?? true);
-      const shouldCreateBlobUrl = isPartialDownload
-        ? false
-        : (params.createBlobUrl ?? false);
-
       const job: DownloadJob = {
         id: `dl-${Date.now()}-${Math.random().toString(36).slice(2, 9)}`,
         assetId: params.assetId,
@@ -173,10 +125,7 @@ class DownloadManagerClass {
         retryCount: 0,
         addedAt: Date.now(),
         storageKey,
-        createBlobUrl: shouldCreateBlobUrl,
-        persist: shouldPersist,
-        maxBytes: params.maxBytes,
-        isPartial: isPartialDownload,
+        persist: params.persist ?? true,
         resolve,
         reject,
       };
@@ -274,39 +223,23 @@ class DownloadManagerClass {
     });
 
     try {
-      // Build request headers - use Range header for partial downloads
-      const headers: HeadersInit = {};
-      if (job.maxBytes) {
-        headers['Range'] = `bytes=0-${job.maxBytes - 1}`;
-        logger.info('[DownloadManager] Partial download requested', {
-          url: job.url.slice(-50),
-          maxBytes: job.maxBytes,
-        });
-      }
-
       const response = await fetch(job.url, {
         signal: job.abortController.signal,
-        headers,
       });
 
-      // Accept both 200 OK and 206 Partial Content
-      if (!response.ok && response.status !== 206) {
+      if (!response.ok) {
         throw new Error(`HTTP ${response.status}: ${response.statusText}`);
       }
 
       const contentLength = response.headers.get('content-length');
       job.totalBytes = contentLength ? parseInt(contentLength, 10) : 0;
 
-      // For partial downloads, track if we reached the limit
-      const isPartialResponse = response.status === 206 || job.maxBytes;
-
       let blob: Blob;
 
       if (response.body) {
         // Stream with progress tracking
         const reader = response.body.getReader();
         const chunks: BlobPart[] = [];
-        let reachedLimit = false;
 
         while (true) {
           const { done, value } = await reader.read();
@@ -315,22 +248,6 @@ class DownloadManagerClass {
           chunks.push(value);
           job.bytesLoaded += value.length;
 
-          // Check if we've reached the maxBytes limit
-          if (job.maxBytes && job.bytesLoaded >= job.maxBytes) {
-            reachedLimit = true;
-            logger.info('[DownloadManager] Reached partial download limit', {
-              bytesLoaded: job.bytesLoaded,
-              maxBytes: job.maxBytes,
-            });
-            // Cancel the remaining download gracefully
-            try {
-              await reader.cancel();
-            } catch {
-              // Ignore cancel errors - stream may already be closed
-            }
-            break;
-          }
-
           job.progress =
             job.totalBytes > 0
               ? Math.round((job.bytesLoaded / job.totalBytes) * 100)
@@ -357,11 +274,6 @@ class DownloadManagerClass {
           type:
             response.headers.get('content-type') || 'application/octet-stream',
         });
-
-        // For partial downloads, mark as complete even if we didn't get everything
-        if (reachedLimit || isPartialResponse) {
-          job.progress = 100; // Consider partial download as "complete"
-        }
       } else {
         // No streaming, get blob directly
         blob = await response.blob();
@@ -371,34 +283,25 @@ class DownloadManagerClass {
       }
 
       // Store the asset using canonical storage key
-      // Partial downloads are now stored with isPartial: true for offline mode awareness
       await StorageManager.storeAsset(job.storageKey, blob, {
         id: job.assetId,
         projectId: job.projectId,
         filename: job.filename,
         variantType: job.variantType,
         assetType: job.assetType,
-        isPartial: job.isPartial || false,
+        isPartial: false,
       });
 
-      if (job.isPartial) {
-        // Mark partial download as ready in session cache
-        this.partialDownloadsReady.add(job.storageKey);
+      // Store ORIGINAL blob directly (bypass cache retrieval to avoid potential corruption)
+      this.readyBlobs.set(job.storageKey, blob);
+      logger.info('[DownloadManager] Stored original blob', {
+        storageKey: job.storageKey.slice(-50),
+        blobSize: blob.size,
+        blobType: blob.type,
+      });
 
-        // Register with Service Worker for full-file caching during playback
-        // When the browser fetches the full media, SW will cache it using the storage key
-        this.registerUrlForCaching(job.url, job.storageKey);
-
-        logger.info('[DownloadManager] Partial download complete (stored)', {
-          storageKey: job.storageKey.slice(-50),
-          bytesLoaded: job.bytesLoaded,
-        });
-      } else {
-        // Create blob URL if requested (full downloads only)
-        if (job.createBlobUrl) {
-          await this.createBlobUrlFromCache(job.storageKey);
-        }
-      }
+      // Always create blob URL for reliable playback on all devices
+      await this.createBlobUrlFromCache(job.storageKey);
 
       // Mark as completed
       job.status = 'completed';
@@ -625,7 +528,6 @@ class DownloadManagerClass {
         retryCount: item.retryCount,
         addedAt: item.addedAt,
         storageKey,
-        createBlobUrl: true, // Create blob URL for resumed downloads
         persist: true,
       };
 
@@ -649,6 +551,16 @@ class DownloadManagerClass {
     return this.readyBlobUrls.get(storageKey) || null;
   }
 
+  /**
+   * Get raw Blob for creating fresh blob URLs (O(1) lookup).
+   * Used by transitions to avoid decoder state issues with pre-created blob URLs.
+   * Returns null if blob not cached.
+   */
+  getReadyBlob(url: string): Blob | null {
+    const storageKey = extractStoragePath(url);
+    return this.readyBlobs.get(storageKey) || null;
+  }
+
   /**
    * Cache an externally fetched blob and register blob URL for instant lookup.
    * Use this when fetching via XHR (e.g., transition playback) to enable caching.
@@ -670,6 +582,9 @@ class DownloadManagerClass {
       assetType: metadata.assetType,
     });
 
+    // Store raw Blob for transitions (they need fresh blob URLs each playback)
+    this.readyBlobs.set(storageKey, blob);
+
     // Create blob URL and register for instant O(1) lookup
     const blobUrl = URL.createObjectURL(blob);
     this.readyBlobUrls.set(storageKey, blobUrl);
@@ -701,6 +616,13 @@ class DownloadManagerClass {
         return;
       }
 
+      // Store raw Blob for transitions (they need fresh blob URLs each playback
+      // to avoid decoder state issues that cause video jumping)
+      // Only set if not already present (original download blob takes priority)
+      if (!this.readyBlobs.has(storageKey)) {
+        this.readyBlobs.set(storageKey, blob);
+      }
+
       const blobUrl = URL.createObjectURL(blob);
 
       // Decode images to prevent white flash
@@ -727,41 +649,13 @@ class DownloadManagerClass {
       });
     }
   }
-
   /**
-   * Register a presigned URL → storage key mapping with the Service Worker.
-   * This enables the SW to cache the full response when the browser fetches the media
-   * during playback, using the canonical storage key instead of the expiring presigned URL.
-   */
-  private registerUrlForCaching(
-    presignedUrl: string,
-    storageKey: string,
-  ): void {
-    if (navigator.serviceWorker?.controller) {
-      navigator.serviceWorker.controller.postMessage({
-        type: 'REGISTER_CACHE_URL',
-        payload: { presignedUrl, storageKey },
-      });
-      logger.info('[DownloadManager] Registered URL for SW caching', {
-        storageKey: storageKey.slice(-40),
-      });
-    }
-  }
-
-  /**
-   * Clear blob URLs and partial downloads cache (call on unmount to prevent memory leaks)
+   * Clear blob URLs cache (call on unmount to prevent memory leaks)
    */
   clearBlobUrls(): void {
     this.readyBlobUrls.forEach((blobUrl) => URL.revokeObjectURL(blobUrl));
     this.readyBlobUrls.clear();
-    this.partialDownloadsReady.clear();
-
-    // Clear SW URL mappings (optional, SW has its own cleanup interval)
-    if (navigator.serviceWorker?.controller) {
-      navigator.serviceWorker.controller.postMessage({
-        type: 'CLEAR_URL_MAPPINGS',
-      });
-    }
+    this.readyBlobs.clear();
   }
 
   /**
@@ -774,7 +668,7 @@ class DownloadManagerClass {
         URL.revokeObjectURL(blobUrl);
         this.readyBlobUrls.delete(key);
       }
-      this.partialDownloadsReady.delete(key);
+      this.readyBlobs.delete(key);
     }
   }
 
diff --git a/frontend/src/pages/constructor.tsx b/frontend/src/pages/constructor.tsx
index b74d0d8..9a9f558 100644
--- a/frontend/src/pages/constructor.tsx
+++ b/frontend/src/pages/constructor.tsx
@@ -601,6 +601,7 @@ const ConstructorPage = ({ mode = 'constructor' }: ConstructorPageProps) => {
       preloadedUrls: preloadOrchestrator.preloadedUrls,
       getCachedBlobUrl: preloadOrchestrator.getCachedBlobUrl,
       getReadyBlobUrl: preloadOrchestrator.getReadyBlobUrl,
+      getReadyBlob: preloadOrchestrator.getReadyBlob,
     },
   });
 
@@ -1831,6 +1832,7 @@ const ConstructorPage = ({ mode = 'constructor' }: ConstructorPageProps) => {
       </div>
 
       <TransitionPreviewOverlay
+        videoKey={transitionPreview?.videoUrl}
         videoRef={transitionVideoRef}
         isActive={Boolean(transitionPreview)}
         isBuffering={isReverseBuffering}
diff --git a/frontend/src/stores/project_transition_settings/projectTransitionSettingsSlice.ts b/frontend/src/stores/project_transition_settings/projectTransitionSettingsSlice.ts
index 507d771..a66d096 100644
--- a/frontend/src/stores/project_transition_settings/projectTransitionSettingsSlice.ts
+++ b/frontend/src/stores/project_transition_settings/projectTransitionSettingsSlice.ts
@@ -72,6 +72,11 @@ export const fetchByProjectAndEnv = createAsyncThunk<
       return { key, data: result.data };
     } catch (error) {
       if (isAxiosError(error) && error.response) {
+        // 404 means no project-specific settings exist - this is expected,
+        // the app will fall back to global defaults
+        if (error.response.status === 404) {
+          return { key, data: null };
+        }
         return rejectWithValue(error.response.data as ApiError);
       }
       throw error;