Bootstrap/scss/_animation.scss

.fade {
  opacity: 0;

  @if $enable-transitions {
    transition: opacity .15s linear;
  }

  &.active {
    opacity: 1;
  }
}

.collapse {
  display: none;
  &.active {
    display: block;
  }
}

tr {
  &.collapse.active {
    display: table-row;
  }
}

tbody {
  &.collapse.active {
    display: table-row-group;
  }
}

.collapsing {
  position: relative;
  height: 0;
  overflow: hidden;

  @if $enable-transitions {
    transition-timing-function: ease;
    transition-duration: .35s;
    transition-property: height;
  }
}