electronotes/app/renderer/common/css/materialize/components/_collapsible.scss

.collapsible {
  border-top: 1px solid $collapsible-border-color;
  border-right: 1px solid $collapsible-border-color;
  border-left: 1px solid $collapsible-border-color;
  margin: $element-top-margin 0 $element-bottom-margin 0;
  @extend .z-depth-1;
}

.collapsible-header {
  &:focus {
    outline: 0
  }

  display: flex;
  cursor: pointer;
  -webkit-tap-highlight-color: transparent;
  line-height: 1.5;
  padding: 1rem;
  background-color: $collapsible-header-color;
  border-bottom: 1px solid $collapsible-border-color;

  i {
    width: 2rem;
    font-size: 1.6rem;
    display: inline-block;
    text-align: center;
    margin-right: 1rem;
  }
}
.keyboard-focused .collapsible-header:focus {
  background-color: #eee;
}

.collapsible-body {
  display: none;
  border-bottom: 1px solid $collapsible-border-color;
  box-sizing: border-box;
  padding: 2rem;
}

// Sidenav collapsible styling
.sidenav,
.sidenav.fixed {

  .collapsible {
    border: none;
    box-shadow: none;

    li { padding: 0; }
  }

  .collapsible-header {
    background-color: transparent;
    border: none;
    line-height: inherit;
    height: inherit;
    padding: 0 $sidenav-padding;

    &:hover { background-color: rgba(0,0,0,.05); }
    i { line-height: inherit; }
  }

  .collapsible-body {
    border: 0;
    background-color: $collapsible-header-color;

    li a {
      padding: 0 (7.5px + $sidenav-padding)
               0 (15px + $sidenav-padding);
    }
  }

}

// Popout Collapsible

.collapsible.popout {
  border: none;
  box-shadow: none;
  > li {
    box-shadow: 0 2px 5px 0 rgba(0, 0, 0, 0.16), 0 2px 10px 0 rgba(0, 0, 0, 0.12);
    // transform: scaleX(.92);
    margin: 0 24px;
    transition: margin .35s cubic-bezier(0.250, 0.460, 0.450, 0.940);
  }
  > li.active {
    box-shadow: 0 5px 11px 0 rgba(0, 0, 0, 0.18), 0 4px 15px 0 rgba(0, 0, 0, 0.15);
    margin: 16px 0;
    // transform: scaleX(1);
  }
}