Fixhancement: version-aware thumbnail etag (#12754)

src/documents/conditionals.py

@@ -117,6 +117,17 @@ def preview_last_modified(request, pk: int) -> datetime | None:
     return doc.modified
 
 
+def thumbnail_etag(request: Any, pk: int) -> str | None:
+    """
+    Thumbnails are version-dependent, so use the effective document checksum as
+    the ETag to invalidate cache when the latest version changes.
+    """
+    doc = resolve_effective_document_by_pk(pk, request).document
+    if doc is None:
+        return None
+    return doc.checksum
+
+
 def thumbnail_last_modified(request: Any, pk: int) -> datetime | None:
     """
     Returns the filesystem last modified either from cache or from filesystem.