func (t *tbfe) VisibleRegion(v *backend.View) Region {
t.lock.Lock()
r, ok := t.layout[v]
t.lock.Unlock()
if !ok || r.lastUpdate != v.ChangeCount() {
t.Show(v, r.visible)
t.lock.Lock()
r = t.layout[v]
t.lock.Unlock()
}
return r.visible
}
