When uploadprogress is available, Drupal.file.progressBar temporarily changes the name of the hidden field that contains the key for pecl (from filename[UPLOAD_IDENTIFIER] to UPLOAD_IDENTIFIER) or apc uploadprogress (from filename[APC_UPLOAD_PROGRESS] to APC_UPLOAD_PROGRESS). However, for some reason, this doesn't end up in the post data send back to the server, so further requests to file/progress/key will not report the progress since it can't find it.
The code in file.js is more or less the same. The only difference is that we now autotrigger a mousedown on the upload button, however, disabling that doesn't fix this problem, so this behavior doesn't seem to be the main problem here.
D7 post data
------WebKitFormBoundary4d4zgU831PUSggVA Content-Disposition: form-data; name="UPLOAD_IDENTIFIER" 123487692
D8 post data
------WebKitFormBoundaryctCyV2fneyPxBbKl Content-Disposition: form-data; name="field_file[UPLOAD_IDENTIFIER]" 426806279
|#45||core-progressbar-fix-2662932-45.patch||598 bytes||Peter Swietoslawski|