chromium/chrome/browser/resources/commerce/product_specifications/new_column_selector.html

<style include="cr-shared-style">
  :host {
    --iron-icon-width: 20px;
    --new-column-selector-border-radius: 10px;
    --new-column-selector-size: 42px;
    position: relative;
    text-align: start;
  }

  #button {
    align-items: center;
    background: var(--color-product-specifications-tonal-button-background);
    display: flex;
    justify-content: center;
  }

  #button,
  #hoverLayer {
    border-radius: var(--new-column-selector-border-radius);
    height: var(--new-column-selector-size);
    width: var(--new-column-selector-size);
  }

  #hoverLayer {
    background-color: var(--cr-hover-background-color);
    display: none;
    position: absolute;
  }

  #productSelectionMenu {
    --product-selector-width: 280px;
  }

  #button:active > #hoverLayer,
  #button:focus-within > #hoverLayer,
  #button:hover > #hoverLayer,
  #button.showing-menu > #hoverLayer {
    display: block;
  }
</style>

<div id="button"
    on-click="showMenu_"
    on-keydown="onButtonKeyDown_"
    tabindex="0">
  <cr-icon icon="cr:add"></cr-icon>
  <div id="hoverLayer"></div>
</div>

<product-selection-menu id="productSelectionMenu"
    excluded-urls="[[excludedUrls]]"
    on-close-menu="onCloseMenu_">
</product-selection-menu>