xstatic-bootstrap-scss/xstatic/pkg/bootstrap_scss/data/scss/bootstrap/_component-animations.scss

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

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

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