我不知道。这似乎工作...（一种hack虽然）。

I don't know. This seems to work... (kind of a hack though).

.navbar-fixed-top {
  top: -70px; /* you'll have to figure out the exact number here */
}

.navbar-fixed-top, .navbar-fixed-bottom {
  position: relative; /* this can also be static */
}