This has happened twice now in the latest version, though I can’t reproduce it yet. I have a script with an open handler, in debug mode, but when I drop a file on the script’s window, SD acts as if it is not aware of the open handler. The Invoke Open Handler option in the sheet is disabled, with only the usual Paste ones below available.
In each case, the solution is to close the script and re-open it. Then all works as designed.