fix(library): keep the shell the deploy replaced out of a failed navigation, and say when another tab holds the library open

docker/frontend/src/Library.tsx

@@ -20,6 +20,7 @@ import {
   listEditedIds,
   listFolders,
   normPath,
+  onLibraryBlocked,
   pickFolder,
   readPhotoFile,
   readPhotos,
@@ -376,6 +377,15 @@ function Catalogue() {
   // the folders and cannot tell whether the remembered subfolder still exists.
   const [loaded, setLoaded] = useState(false);
 
+  // Whether another tab still holds the library database at the version it was
+  // opened with — a tab left open across a build. Nothing can be read until it
+  // goes, and the screen would otherwise sit blank with no reason on it.
+  const [libraryBlocked, setLibraryBlocked] = useState(false);
+  useEffect(() => {
+    onLibraryBlocked(setLibraryBlocked);
+    return () => onLibraryBlocked(null);
+  }, []);
+
   // The folder the catalogue is copied into, and whether the browser is still
   // letting this page write to it — which it stops doing when the tab closes, so
   // the row below says so rather than letting a write fail quietly. The ref is
@@ -1614,6 +1624,12 @@ function Catalogue() {
               ) : null}
             </div>
 
+            {libraryBlocked ? (
+              <p className="hint adm-note" data-key="lib-blocked">
+                {t('lib.blocked')}
+              </p>
+            ) : null}
+
             {/* The folders are on their way in for as long as the catalogue behind
                 them is being read back, and a screen that says there is no folder
                 before it has looked is a screen that says it every visit. */}