I've tried in assignment > edit submission > upload a file.
I drag a file into the upload area.
During the upload, before it's completed, I click save (eg by accident)
An error appears 'do you want to leave or stay on the page, I click stay on the page
The save button becomes greyed out with a 'disabled' tag
It continues uploading and completes
The disabled tag does not get removed, so you cannot save.
<input type="submit" class="btn btn-primary" name="submitbutton" id="id_submitbutton" value="Save changes" disabled="">