style.css

/**
 * Button state:
 *    1. normal (class name: [none])
 *    2. opening (class name: opening)
 *    3. opened (class name: opened)
 *    4. closing (class name: closing)
 *    5. closed (class name: closed)
 *    6. hiding (class name: hiding)
 *    8. flying (class name: flying)
**/

/**
 * Normal:
 *   open: closed -> opening -> opened
 *   close: opened -> closing -> closed
 *   hide: current state -> hiding -> closed
 *   flyOpen: flying -> closed -> opening -> opened
**/

smart-banner {
  outline: 0;
  display: inline-block;
  height: 7rem;
  /* Same as closed state */
  padding-left: 11rem;
  padding-right: 0;
  padding-top: 2rem;
  padding-bottom: 2rem;
  opacity: 1;
  /* we need to set border to none to remove the button border */
  border: none;
  /* the radius is still make button as round even if we don't have border */
  border-radius: 5.5rem;
  background-color: rgba(0, 0, 0, 0.5);
  background-repeat: no-repeat;
  background-size: 5.4rem auto;
  background-position: 2.8rem 50%;

  word-break: keep-all;
  white-space: nowrap;
  overflow: hidden;
  position: relative;

  font-size: 2.8rem;
  line-height: 7rem;

  transition-property: none;
  transition-timing-function: cubic-bezier(0.25, 0, 0, 1.0);
  transition-duration: 0s;
}

smart-banner:before {
  content: " ";
  background-repeat: no-repeat;
  background-size: 5.4rem auto;
  background-position: center;
  width: 5.4rem;
  height: 7rem;
  display: inline-block;
  position: absolute;
  left: 2.8rem;

  transition-property: all;
  transition-timing-function: cubic-bezier(0.25, 0, 0, 1.0);
  transition-duration: 0.42s;
}

smart-banner.closed:before {
  width: 11rem;
  left: 0;
}

smart-banner[data-icon]:before {
  width: 5.4rem;
  left: 4.4rem;
  font-size: 5.4rem;
  line-height: 11rem;
  text-align: center;
  color: #5f6368;
  white-space: normal;
  top: 0;
}

smart-banner[data-icon].closed:before {
  left: 2.8rem;
}

smart-banner.closed {
  padding-left: 11rem;
  padding-right: 0;
  transition-duration: 0.5s;
  background-color: rgba(0, 0, 0, 0.5);
}

smart-banner.closed > * {
  display: none;
}

smart-banner.opened {
  padding-right: 4.4rem;
}

smart-banner.opening {
  transition-property: all;
  transition-duration: 0.72s;
  /* add the same padding-right of normal to make it run transition */
  padding-right: 4.4rem;
}

smart-banner.opening:before {
  transition-duration: 0.72s;
}

smart-banner.closing {
  transition-property: all;
  transition-duration: 0.5s;
  padding-left: 11rem;
  padding-right: 0;
  background-color: rgba(0, 0, 0, 0.5);
}

smart-banner.hiding {
  opacity: 0;
  transition-property: opacity;
  transition-duration: 1s;
}

smart-banner.flying {
  animation: flyAnimation 1s;
}

@keyframes flyAnimation {
    from {transform: translateY(20rem);}
    to {transform: translateY(0);}
}