Linux Mac OSX Windows 
 Interfaces for overriding the built-in drag, drop, copy, and paste
 implementations in the content area and editors. Use this to do things
 such as prevent a drag from starting, adding or removing
 data and flavors, or preventing the drop.

 Embedders who want to have these hooks made available should implement
 nsIClipboardDragDropHooks and use the command manager to send the
 appropriate commands with these parameters/settings: 
      command:  cmd_clipboardDragDropHook

      params        value type   possible values
      "addhook"     isupports    nsIClipboardDragDropHooks as nsISupports
      "removehook"  isupports    nsIClipboardDragDropHooks as nsISupports

 Notes:
  * Overrides/hooks need to be added to each window (as appropriate).
    Adding them to the first window does not enable them for every window.
  * If more than one implementation is set for a window, the hooks will be
    called in the order they are added.
  * Adding the same hook to the same window will not add a second call.
    Each hook can only be called once per user action/api.
  * Not all hooks are guaranteed to be called.  If there are multiple hooks
    set for a window, any of them has an opportunity to cancel the action
    so no further processing will occur.
  * If any errors occur (without setting the boolean result) the default
    action will occur.
  * AllowDrop will be called MANY times during drag so ensure that it is
    efficient.
[scriptable, uuid(e03e6c5e-0d84-4c0b-8739-e6b8d51922de)]
interface nsIClipboardDragDropHooks : nsISupports

Methods

 Tells gecko whether a drop is allowed on this content area

 @param event   DOM event (drag over)
 @param session the drag session from which client can get
                   the flavors present or the actual data

 @return TRUE  indicates to OS that if a drop does happen on this
                browser, it will be accepted.
 @return FALSE indicates to OS drop is not allowed. On win32, this
                will change the cursor to "reject".
boolean allowDrop(in nsIDOMEvent event, in nsIDragSession session) 
 Prevents the drag from starting

 @param event DOM event (drag gesture)

 @return TRUE  drag can proceed
 @return FALSE drag is cancelled, does not go to OS
boolean allowStartDrag(in nsIDOMEvent event) 
 Alter the flavors or data presented to the OS
 Used for drag and copy actions
 Because this can be called many times, it is highly recommended
 that the implementation be very efficient so user feedback is
 not negatively impacted.

 @param event  DOM event (drag drop); null if triggered by copy.
 @param trans  the transferable holding the list of flavors
               and the data for each flavor

 @return TRUE  copy/drag can proceed
 @return FALSE copy/drag is cancelled, does not go to OS
boolean onCopyOrDrag(in nsIDOMEvent aEvent, in nsITransferable trans) 
 Provide an alternative action to the built-in behavior when
 something is dropped on the browser or in an editor

 @param event  DOM event (drag drop); null if triggered by paste.
 @param trans  the transferable holding the list of flavors
               and the data for each flavor

 @return TRUE  action was handled, do not perform built-in
                behavior
 @return FALSE action was not overridden, do built-in behavior
boolean onPasteOrDrop(in nsIDOMEvent event, in nsITransferable trans)