Source

yii-bootstrap2 / bootstrap / vendors / bootstrap / less / component-animations.less

// COMPONENT ANIMATIONS
// --------------------

.fade {
  .transition(opacity .15s linear);
  opacity: 0;
  &.in {
    opacity: 1;
  }
}

.collapse {
  .transition(height .35s ease);
  position:relative;
  overflow:hidden;
  height: 0;
  &.in { height: auto; }
}