metamask-extension/ui/components/component-library/popover/popover.scss

.mm-popover {
  box-shadow: var(--shadow-size-md) var(--color-shadow-default);

  /* Hide the popper when the reference is hidden */
  &--reference-hidden[data-popper-reference-hidden="true"] {
    visibility: hidden;
    pointer-events: none;

    > .mm-popover__arrow::before {
      visibility: hidden;
    }
  }
}

.mm-popover__arrow,
.mm-popover__arrow::before {
  position: absolute;
  width: 8px;
  height: 8px;
  background: inherit;
}

.mm-popover__arrow {
  width: 40px;
  height: 40px;
  visibility: hidden;
}

.mm-popover__arrow::before {
  display: block;
  background-color: inherit;
  border: 1px solid;
  border-left-color: inherit;
  border-top-color: inherit;
  border-bottom-color: transparent;
  border-right-color: transparent;
  visibility: visible;
  content: '';
  transform: rotate(45deg);
  border-radius: 2px  0 0 0;
}

.mm-popover[data-popper-placement^='top'] > .mm-popover__arrow {
  bottom: -20px;

  &::before {
    transform: rotate(-135deg);
  }
}

.mm-popover[data-popper-placement^='bottom'] > .mm-popover__arrow {
  top: -20px;
}

.mm-popover[data-popper-placement^='left'] > .mm-popover__arrow {
  right: -20px;

  &::before {
    transform: rotate(135deg);
  }
}

.mm-popover[data-popper-placement^='right'] > .mm-popover__arrow {
  left: -20px;

  &::before {
    transform: rotate(-45deg);
  }
}