[Bug]: Folder preview thumbnails permanently show loading spinner for newly uploaded assets Module

[Refaat-alktifan]

Version

2.3.2 (also affects 2.2.x, 2.3.0, 2.3.1)

Description

When uploading images to an asset folder and viewing them in the Preview tab, thumbnails that haven't been generated yet show a loading spinner (video-loading.gif) that never resolves, even after the thumbnail has been generated by the messenger worker. The spinner persists until the user does a hard browser refresh (Ctrl+F5) or switches to the List view.

Root Cause

Two issues in AssetController::getImageThumbnailAction() at line 1179-1181:

1. Placeholder response is cacheable

When origin=folderPreview and the thumbnail doesn't exist yet, the controller returns video-loading.gif as a BinaryFileResponse without Cache-Control: no-store headers. The browser caches this response.

The getFolderContentPreviewAction generates thumbnail URLs using the asset's modificationDate as the _dc cache-buster parameter (e.g. _dc=1774887063). Since this timestamp never changes for the same asset, subsequent requests hit the browser cache and serve the cached spinner forever.

2. No async thumbnail generation is dispatched

For origin=treeNode, the controller dispatches AssetPreviewImageMessage to trigger async thumbnail generation. But for origin=folderPreview, no message is dispatched — the thumbnail is only generated if/when the pimcore_asset_update worker processes it independently.

Steps to Reproduce

1. Open the Pimcore admin UI
2. Create a new asset folder
3. Upload 5-6 images (e.g. JPEG, 3-600KB each)
4. Open the folder → click Preview tab
5. Observe: some images show the loading spinner, others may show correctly (depends on timing)
6. Wait 30+ seconds (worker generates thumbnails)
7. Reload the page → spinners persist
8. Switch to List view → all thumbnails display correctly

Why List view works

The List view uses a different URL pattern:

/admin/asset/get-image-thumbnail?id=1429852&treepreview=1&width=108&height=70&frame=1

The Preview (folder) view uses:

/admin/asset/get-image-thumbnail?id=1429852&treepreview=1&_dc=1774887063&origin=folderPreview

Different URL = no cached stale response in the browser.

Current code

https://github.com/pimcore/admin-ui-classic-bundle/blob/2.3/src/Controller/Admin/Asset/AssetController.php#L1169-L1183

Proposed fix

} elseif ($request->get('origin') === 'folderPreview') {
    \Pimcore::getContainer()->get('messenger.bus.pimcore-core')->dispatch(
        new AssetPreviewImageMessage($image->getId())
    );
    $response = new BinaryFileResponse(PIMCORE_WEB_ROOT . '/bundles/pimcoreadmin/img/video-loading.gif');
    $response->headers->set('Cache-Control', 'no-store, must-revalidate');
    $response->headers->set('Pragma', 'no-cache');
    $response->headers->set('Expires', '0');
    return $response;
}

Changes:

1. Dispatch AssetPreviewImageMessage : triggers async thumbnail generation (same behavior as treeNode)
2. Set Cache-Control: no-store : prevents the browser from caching the placeholder spinner, so the next request hits the server and gets the real thumbnail