The Delete Draft command will delete the draft that the application saved the current message as a draft and displays in the editor just in case you’d like to complete your message later (for 24 hours) or you logged out due to session timeout. However, it doesn’t clear the editor box content.
If you click Delete Draft and keep the same window open without clearing the editor box (selecting all, deleting the content or type something else) then it will re-save the existing content as a draft in the next 60 seconds. So, it will be a saved draft again.
Instead of closing the browser, you can refresh the page right after you click to Delete Draft if you don’t prefer to select all and delete. It will make sure the editor and its memory is cleared.