chrome/browser/resources/pdf/elements/viewer-toolbar-dropdown.html

    <style>
      :host {
        display: inline-block;
        position: relative;
        text-align: start;
        --dropdown-width: 260px;
      }

      :host-context([hidden]) {
        display: none;
      }

      :host([dropdown-centered]) {
        --container-offset: calc(50% - var(--dropdown-width) / 2);
      }

      #container {
        position: absolute;
        right: var(--container-offset, 0);
      }

      :host-context([dir=rtl]) #container {
        left: var(--container-offset, 0);
        right: auto;
      }

      #dropdown {
        background-color: var(--cr-menu-background-color);
        border-radius: 4px;
        box-shadow: var(--cr-menu-shadow);
        color: var(--cr-primary-text-color);
        overflow-y: hidden;
        padding-bottom: 2px;
        width: var(--dropdown-width);
      }

      @media (prefers-color-scheme: dark) {
        #dropdown {
          background-image: linear-gradient(var(--cr-menu-background-sheen),
                                            var(--cr-menu-background-sheen));
        }
      }

      #scroll-container {
        max-height: 300px;
        overflow-y: auto;
        padding: 6px 0 4px 0;
      }

      #button {
        --cr-icon-button-fill-color: currentColor;
        --cr-icon-button-height: 32px;
        --cr-icon-button-icon-start-offset: 4px;
        --cr-icon-button-width: 48px;
        margin: 0;
      }

      #button:hover {
        background-color: rgba(255, 255, 255, 0.08);
        opacity: 1;
      }

      :host([selected]) #button {
        opacity: 1;

      }
      :host([dropdown-open]) #button {
        background-color: var(--active-button-bg);
      }

      h1 {
        border-bottom: var(--cr-separator-line);
        font-size: 0.87rem;
        font-weight: 500;
        margin: 0;
        padding: 14px 28px;
      }
    </style>
    <cr-icon-button on-click="toggleDropdown" id="button"
        iron-icon="[[dropdownIcon_]],cr:arrow-drop-down" title$="[[header]]">
    </cr-icon-button>

    <div id="container">
      <div id="dropdown" style="display: none">
        <template is="dom-if" if="[[!hideHeader]]">
          <h1>[[header]]</h1>
        </template>
        <div id="scroll-container">
          <slot></slot>
        </div>
      </div>
    </div>