VBF_HISTORY_FULL
vb->vb_flags |= VBF_HISTORY_FULL;
vb->vb_flags &= ~VBF_HISTORY_FULL;
history_full = vb->vb_flags & VBF_HISTORY_FULL ||
vb->vb_flags &= ~VBF_HISTORY_FULL;
vb->vb_flags |= VBF_HISTORY_FULL;
top = (vb->vb_flags & VBF_HISTORY_FULL) ?