zen-desktop/src/browser/base/content/zen-styles/zen-folders.css

tab-group[split-view-group] {
  display: flex;
}

tab-group[split-view-group] > .tab-group-container {
  display: flex;
  flex: 1;
  flex-wrap: nowrap;
  border-radius: var(--border-radius-medium);
  padding: 0 2px;
  margin-inline: var(--tab-block-margin);
  margin-block: var(--tab-block-margin);
  min-height: var(--tab-min-height);
  outline: var(--tab-outline);
  outline-offset: var(--tab-outline-offset);
  outline-color: var(--tab-selected-outline-color);
  transition: scale 0.1s ease;
  align-items: center;
  --zen-split-view-active-tab-bg: color-mix(in srgb, var(--zen-toolbar-element-bg), transparent 40%);

  :root:not([zen-sidebar-expanded='true']) & {
    padding: 0 2px;
    --tab-min-height: 30px;
    --tab-collapsed-width: 38px;
    margin: 0 4px;
    --tab-min-width: 34px;
  }

  & > .tabbrowser-tab {
    --tab-selected-bgcolor: var(--zen-split-view-active-tab-bg);
    --tab-hover-background-color: transparent;
    --tab-selected-shadow: none;
    --border-radius-medium: var(--tab-border-radius);
    --zen-active-tab-scale: 1;

    :root[zen-sidebar-expanded='true'] & {
      --tab-min-height: 28px;
    }

    container-type: inline-size;
    container-name: browser-tab;

    flex: 1 !important;
    padding-inline: 2px !important;
    overflow: clip;

    &:not(:last-child)::after {
      content: '';
      width: 1px;
      height: 16px;
      background-color: light-dark(rgba(0, 0, 0, 0.1), rgba(255, 255, 255, 0.2));
      position: absolute;
      right: 0;
      top: 50%;
      transform: translateY(-50%);
    }

    :root:not([zen-sidebar-expanded='true']) &:not(:last-child)::after {
      width: 16px;
      height: 1px;
      top: auto;
      bottom: 0;
      right: 50%;
      transform: translateX(50%);
    }

    & .tab-content {
      min-width: 0;
      :root[zen-sidebar-expanded='true'] & {
        justify-content: unset !important;
      }
    }
  }

  &:has(> tab:is([visuallyselected], [multiselected])) {
    background-color: var(--tab-selected-bgcolor);
    box-shadow: var(--tab-selected-shadow);

    & > .tabbrowser-tab {
      --tab-hover-background-color: var(--zen-split-view-active-tab-bg);
      & .tab-background {
        background-color: var(--zen-split-view-active-tab-bg) !important;
      }

      &::after {
        display: none;
      }
    }
  }

  &:active {
    scale: var(--zen-active-tab-scale);
  }

  &:hover {
    background-color: var(--zen-toolbar-element-bg);
  }

  & .tab-close-button,
  & .tab-reset-button {
    margin-inline-end: -3px !important;
    display: none !important;
  }

  @container browser-tab (min-width: 70px) {
    :root[zen-sidebar-expanded='true'] &:hover > .tabbrowser-tab:not([pinned]) .tab-close-button {
      display: block !important;
    }
  }

  @media (prefers-reduced-motion: no-preference) {
    #tabbrowser-tabs[movingtab] & {
      transition: var(--tab-dragover-transition);
    }
  }
}

:root:not([zen-sidebar-expanded='true']) {
  tab-group {
    flex-direction: column;
  }
}

tab-group[split-view-group] .tabbrowser-tab {
  width: 100%;
  max-width: unset;
}

tab-group[split-view-group] .tab-group-label-container {
  visibility: collapse;
}

tab-group[split-view-group] .tab-close-button {
  display: block;
  visibility: visible;
}

tab-group[split-view-group] .tab-group-line {
  display: none;
  background: transparent;
}

.tab-group-line {
  display: none !important;
}

tab-group:not([split-view-group]) {
  display: flex !important;
  flex-direction: column !important;
  @media not (-moz-bool-pref: 'zen.view.sidebar-expanded') {
    width: var(--tab-min-width) !important;
    margin: var(--tab-block-margin) auto !important;
  }

  & > .tab-group-container {
    padding-top: var(--tab-block-margin);
    padding-left: var(--zen-folder-indent, 0.5em);

    & > tab::before {
      background: none !important;
    }
  }

  margin: var(--tab-block-margin);

  & > .tab-group-label-container {
    flex: 0 0 auto !important;
    position: sticky !important;
    top: 0 !important;
    z-index: 1000 !important;
    --tab-group-color-pale: transparent !important;
    --tab-group-color: transparent !important;
    margin: 0 !important;
    height: 36px !important;
    border-radius: var(--border-radius-medium) !important;
    transition: transform 0.2s ease;
    transition:
      background-color 0.2s ease,
      border-radius 0.2s ease-in-out;
    padding-inline: var(--tab-group-label-padding);
    align-items: center;
    @media not (-moz-bool-pref: 'zen.view.sidebar-expanded') {
      width: var(--tab-min-width) !important;
    }

    .tab-group-folder-icon {
      width: 18px;
      height: 18px;
      margin-inline-end: var(--toolbarbutton-inner-padding) !important;
      margin-inline-start: calc(var(--toolbarbutton-inner-padding) / 4) !important;
    }

    &:hover {
      background-color: var(--tab-hover-background-color) !important;
    }

    &:after {
      display: none;
    }

    & > label {
      width: 100% !important;
      background: transparent !important;
      border: none !important;
      color: var(--sidebar-text-color) !important;
      margin: 0 !important;
      padding-left: 34.5px !important;
      font-weight: 500;
      padding: 0 !important;
      align-self: center !important;
      text-align: start;
    }
  }

  &[collapsed] {
    & > .tabbrowser-tab:not([hidden]) {
      display: flex;
    }

    & > .tab-group-container {
      overflow-y: hidden;
    }
  }
}