discourse/app/assets/stylesheets/common/components/footer-nav.scss

// --------------------------------------------------
// Footer nav bar (displayed in DiscourseHub app and PWAs)
// --------------------------------------------------

$footer-nav-height: 49px;

body.footer-nav-visible {
  #main-outlet {
    padding-bottom: $footer-nav-height + 15;
  }

  #reply-control.draft {
    bottom: $footer-nav-height;
    margin-bottom: env(safe-area-inset-bottom);
    padding-bottom: 0px;
  }

  #topic-progress-wrapper:not(.docked) {
    margin-bottom: calc(#{$footer-nav-height} + env(safe-area-inset-bottom));
  }
  .posts-filtered-notice {
    transition: all linear 0.1s;
    bottom: calc(#{$footer-nav-height} + env(safe-area-inset-bottom));
  }
}

.footer-nav {
  background-color: rgba(var(--header_background-rgb), 0.9);
  box-shadow: shadow("footer-nav");
  height: $footer-nav-height;
  position: fixed;
  bottom: -$footer-nav-height;
  left: 0;
  width: 100%;
  z-index: z("footer-nav");
  transition: all linear 0.1s;

  .d-icon {
    color: var(--header_primary-medium);
  }

  &.visible {
    bottom: 0px;
  }

  .footer-nav-widget {
    display: flex;
    justify-content: "space-evenly";
    @include unselectable;
    button {
      flex: 1;
      margin: 4px;
      border-radius: 2px;
      padding: 14px 18px;
      &:disabled {
        opacity: 0.6;
      }
    }
  }
}

@supports (padding: max(0px)) {
  .footer-nav.visible {
    padding-bottom: unquote("max(5px, env(safe-area-inset-bottom))");
  }
}

@supports (-webkit-backdrop-filter: blur(10px)) {
  body:not(.footer-nav-ipad) .footer-nav {
    background-color: rgba(var(--header_background-rgb), 0.7);
    -webkit-backdrop-filter: blur(20px);
  }
}

body.footer-nav-ipad {
  padding-top: $footer-nav-height;
  .footer-nav {
    bottom: auto;
    top: 0px;
    background-color: var(--header_background);
    z-index: z("ipad-header-nav");
    padding-bottom: 0; // resets safe-area-inset-bottom
  }

  #reply-control,
  #reply-control.fullscreen {
    z-index: z("ipad-header-nav") + 1;
  }

  &.docked .d-header-wrap {
    top: $footer-nav-height;
  }
}