I am trying to show to user if "File cannot be dropped to the desired location" on my UI.
Refer to the image attached above, user can drag and drop a file to the rectangular areas defined but not all files can be dropped to any of these rectangle. If user tries to drop a file to a rectangle where it cannot be dropped, I want to show visually to the user that this file cannot be dropped here. For this I am currently showing a red error icon along with file icon while it is being dragged but I want to have a better indication for this. I could not find any content on this on Apple UI Guidelines. Please guide me what other could be options to show if file cannot be dragged to a location.