chromium/chrome/browser/resources/ash/settings/device_page/customize_button_select.html

<style include="md-select settings-shared input-device-settings-shared">
  #menuContainer {
    display: table;
    width: 200px;
  }

  #remappingContainer {
    display: flex;
    gap: 8px;
    width: calc(100% - 15px);
    overflow: scroll;
  }

  iron-dropdown {
    background-color: var(--cros-bg-color-elevation-3);
    border-radius: 2px;
    box-shadow: var(--cr-elevation-3);
    max-height: 50vh;
    overflow: auto;
  }

  .label-container {
    display: flex;
    gap: 8px;
    line-height: 16px;
  }

  .label-container > iron-icon {
    --iron-icon-height: 16px;
    --iron-icon-width: 16px;
  }

  .md-select {
    background-position-x: calc(100% - 15px);
    padding: 10px 16px;
    width: 168px;
  }
</style>
<div tabindex="0" id="selectDropdown" role="button"
    class="md-select label-container"
    on-click="showDropdownMenu_">
  <div id="remappingContainer">
    <template is="dom-if" if="[[remappedToKeyCombination_]]">
      <template is="dom-repeat" items="[[inputKeys_]]">
        <template is="dom-if" if="[[getIconIdForKey_(item)]]">
          <div aria-label="[[getAriaLabelForIcon(item)]]" role="img">
            <iron-icon icon="[[getIconIdForKey_(item)]]"
                aria-hidden="true">
            </iron-icon>
          </div>
        </template>
        <template is="dom-if" if="[[!getIconIdForKey_(item)]]">
          <span>[[item]]</span>
        </template>
      </template>
    </template>
    <template is="dom-if" if="[[!remappedToKeyCombination_]]">
      [[label_]]
    </template>
  </div>
</div>
<iron-dropdown opened="[[shouldShowDropdownMenu_]]"
    no-cancel-on-outside-click
    focus-target="[[focusTarget_]]"
    dynamic-align>
  <div id="menuContainer" slot="dropdown-content">
    <template is="dom-repeat" items="[[menu]]">
      <customize-button-dropdown-item option="[[item]]"
          selected="[[isItemSelected_(item, selectedItem)]]">
      </customize-button-dropdown-item>
    </template>
  </div>
</iron-dropdown>