In some cases, when multiple files are uploaded and the ProgressDialog opens, it closes but a gray veil remains. After reloading the page, the veil is gone again and the upload was successful.
The FolderControl gets a ClientAction that calls DragAndDropFile.hideProgressDialog on the client side once the upload is complete. However, it seems that every now and then the controlId changes in the process. This causes the DragAndDropFile.hideProgressDialog function to break because it can't find anything with document.getElementById(controlId) and then has a NullPointerException. Since hideProgressDialog first removes the progressDialog and then the gray background, the latter is no longer executed due to the error, although it does not depend on the controller.
Improvement
In DragAndDropFile.hideProgressDialog it is now checked whether document.getElementById(controlId) returns null. If it does, the function is aborted. To remove the gray veil anyway, it is removed before the check instead of after as originally.
Test
No test.