In gtkaction.c, function closure_accel_activate we have the bit below.
Note, that it claims to have handled the accellerator even if it did not
really do so.  That prevents the key from being seen by widgets.

(It's not particular good UI, but XL has a global F4=Repeat accellerator and
a cell-edit-only override.)

{
  if (gtk_action_is_sensitive (GTK_ACTION (closure->data)))
    _gtk_action_emit_activate (GTK_ACTION (closure->data));

  /* we handled the accelerator */
  g_value_set_boolean (return_value, TRUE);
}