Fix race conditions in handling of unsupported drops on X

* lisp/x-dnd.el (x-dnd-handle-unsupported-drop): Adjust for new
parameters.

* src/keyboard.c (kbd_buffer, kbd_fetch_ptr, kbd_store_ptr):
Export variables.
(kbd_buffer_get_event): Ignore already handled unsupported
drops.
* src/keyboard.h: Update prototypes.

* src/termhooks.h (enum event_kind): Document meaning of
`modifiers' in UNSUPPORTED_DROP_EVENTs.

* src/xterm.c (x_dnd_send_unsupported_drop): Set event modifiers
to current level.
(x_toggle_visible_pointer): Fix fixes fallback.
(x_dnd_begin_drag_and_drop): Handle UNSUPPORTED_DROP_EVENTs
already in the keyboard buffer before starting DND.
(syms_of_xterm): Give timestamp to unsupported drop function.

* src/xterm.h: Update prototypes.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/termhooks.h b/src/termhooks.h
index 8c193914ba8..08bde0aec0d 100644
--- a/src/termhooks.h
+++ b/src/termhooks.h
@@ -223,6 +223,11 @@ enum event_kind
                                   gives the timestamp where the drop
                                   happened.
 
+                                  .modifiers gives a number that
+                                  determines if an event was already
+                                  handled by
+                                  `x_dnd_begin_drag_and_drop'.
+
                                   .x and .y give the coordinates of
                                   the drop originating from the root
                                   window.  */