css/docs/content/components/select-menu.md

title	status	source	bundle
Select Menu	New	https://github.com/primer/css/tree/master/src/select-menu	select-menu

The SelectMenu component provides advanced support for navigation, filtering, and more. Any menu can make use of JavaScript-enabled live filtering, selected states, tabbed lists, and keyboard navigation with a bit of markup.

Basic example

Use a <details> element to toggle the Select Menu. The <summary> element can be styled in many ways. In the example below it's a .btn.

<details class="details-reset details-overlay" open>
  <summary class="btn" aria-haspopup="true">
    Choose an item
  </summary>
  <div class="SelectMenu">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Title</h3>
        <button class="SelectMenu-closeButton" type="button">
          <!-- <%= octicon "x" %> -->
          <svg width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
            />
          </svg>
        </button>
      </header>
      <div class="SelectMenu-list">
        <button class="SelectMenu-item" role="menuitem">Item 1</button>
        <button class="SelectMenu-item" role="menuitem">Item 2</button>
        <button class="SelectMenu-item" role="menuitem">Item 3</button>
      </div>
    </div>
  </div>
</details>

<div class="d-sm-none" style="height: 600px"><!-- min height for < sm --></div>
<div class="d-none d-sm-block" style="height: 180px"><!-- min height for > sm --></div>

Add a .SelectMenu-header to house a clear title and a close button. Note that the close button is only shown on narrow screens (mobile).

Right aligned

In case the Select Menu should be aligned to the right, use SelectMenu right-0.

<div class="d-flex flex-justify-end position-relative">
  <details class="details-reset details-overlay" open>
    <summary class="btn" aria-haspopup="true">
      Choose an item
    </summary>
    <div class="SelectMenu right-0">
      <div class="SelectMenu-modal">
        <header class="SelectMenu-header">
          <h3 class="SelectMenu-title">Title</h3>
          <button class="SelectMenu-closeButton" type="button">
            <!-- <%= octicon "x" %> -->
            <svg width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
              <path
                fill-rule="evenodd"
                d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
              />
            </svg>
          </button>
        </header>
        <div class="SelectMenu-list">
          <button class="SelectMenu-item" role="menuitem">Item 1</button>
          <button class="SelectMenu-item" role="menuitem">Item 2</button>
          <button class="SelectMenu-item" role="menuitem">Item 3</button>
        </div>
      </div>
    </div>
  </details>
</div>

<div class="d-sm-none" style="height: 600px"><!-- min height for < sm --></div>
<div class="d-none d-sm-block" style="height: 180px"><!-- min height for > sm --></div>

Selected state

Add a .SelectMenu-icon .octicon-check icon and it will show up when aria-checked="true" is set.

<details class="details-reset details-overlay" open>
  <summary class="btn" aria-haspopup="true">
    Choose an item
  </summary>
  <div class="SelectMenu">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Title</h3>
        <button class="SelectMenu-closeButton" type="button">
          <!-- <%= octicon "x" %> -->
          <svg width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
            />
          </svg>
        </button>
      </header>
      <div class="SelectMenu-list">
        <button class="SelectMenu-item" role="menuitemcheckbox" aria-checked="true">
          <!-- <%= octicon "check", class: "SelectMenu-icon" %> -->
          <svg
            width="12"
            height="16"
            viewBox="0 0 12 16"
            class="octicon octicon-check SelectMenu-icon"
            aria-hidden="true"
          >
            <path fill-rule="evenodd" d="M12 5l-8 8-4-4 1.5-1.5L4 10l6.5-6.5L12 5z" />
          </svg>
          Selected state
        </button>
        <button class="SelectMenu-item" role="menuitemcheckbox">
          <!-- <%= octicon "check", class: "SelectMenu-icon" %> -->
          <svg
            width="12"
            height="16"
            viewBox="0 0 12 16"
            class="octicon octicon-check SelectMenu-icon"
            aria-hidden="true"
          >
            <path fill-rule="evenodd" d="M12 5l-8 8-4-4 1.5-1.5L4 10l6.5-6.5L12 5z" />
          </svg>
          Default state
        </button>
        <button class="SelectMenu-item" role="menuitemcheckbox" aria-checked="true">
          <!-- <%= octicon "check", class: "SelectMenu-icon" %> -->
          <svg
            width="12"
            height="16"
            viewBox="0 0 12 16"
            class="octicon octicon-check SelectMenu-icon"
            aria-hidden="true"
          >
            <path fill-rule="evenodd" d="M12 5l-8 8-4-4 1.5-1.5L4 10l6.5-6.5L12 5z" />
          </svg>
          Selected state
        </button>
        <button class="SelectMenu-item" role="menuitemcheckbox">
          <!-- <%= octicon "check", class: "SelectMenu-icon" %> -->
          <svg
            width="12"
            height="16"
            viewBox="0 0 12 16"
            class="octicon octicon-check SelectMenu-icon"
            aria-hidden="true"
          >
            <path fill-rule="evenodd" d="M12 5l-8 8-4-4 1.5-1.5L4 10l6.5-6.5L12 5z" />
          </svg>
          Default state
        </button>
        <button class="SelectMenu-item" role="menuitemcheckbox">
          <!-- <%= octicon "check", class: "SelectMenu-icon" %> -->
          <svg
            width="12"
            height="16"
            viewBox="0 0 12 16"
            class="octicon octicon-check SelectMenu-icon"
            aria-hidden="true"
          >
            <path fill-rule="evenodd" d="M12 5l-8 8-4-4 1.5-1.5L4 10l6.5-6.5L12 5z" />
          </svg>
          Default state
        </button>
      </div>
    </div>
  </div>
</details>

<div class="d-sm-none" style="height: 600px"><!-- min height for < sm --></div>
<div class="d-none d-sm-block" style="height: 250px"><!-- min height for > sm --></div>

List items

The list of items is arguably the most important subcomponent within the menu. Build them out of anchors, buttons, or just about any interactive content. List items are also customizable with options for avatars, additional icons, and multiple lines of text. Use utility classes like mr-2, d-flex or float-right in case more custom styling is needed.

<details class="details-reset details-overlay" open>
  <summary class="btn" aria-haspopup="true">
    Choose an item
  </summary>
  <div class="SelectMenu">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Title</h3>
        <button class="SelectMenu-closeButton" type="button">
          <!--%= octicon "x" %-->
          <svg version="1.1" width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
            />
          </svg>
        </button>
      </header>
      <div class="SelectMenu-list">
        <button class="SelectMenu-item" role="menuitem">
          Text only
        </button>
        <button class="SelectMenu-item" role="menuitem">
          <img
            class="avatar avatar-small mr-2"
            src="https://avatars.githubusercontent.com/hubot?s=40"
            alt="hubot"
            height="20"
            width="20"
          />
          With an avatar
        </button>
        <button class="SelectMenu-item flex-justify-between" role="menuitem">
          With a status icon
          <!-- <%= octicon "primitive-dot", class: "color-green-5 ml-2" %> -->
          <svg
            width="8"
            height="16"
            viewBox="0 0 8 16"
            class="octicon octicon-primitive-dot color-green-5 ml-2"
            aria-hidden="true"
          >
            <path fill-rule="evenodd" d="M0 8c0-2.2 1.8-4 4-4s4 1.8 4 4-1.8 4-4 4-4-1.8-4-4z" />
          </svg>
        </button>
        <button class="SelectMenu-item d-block" role="menuitem">
          With a <span class="Label bg-blue" title="Label: label">label</span>
        </button>
        <button class="SelectMenu-item" role="menuitem">
          With a counter <span class="Counter bg-gray-2 ml-1">16</span>
        </button>
        <button class="SelectMenu-item d-block" role="menuitem">
          <h5>With a heading</h5>
          <span>and some longer description</span>
        </button>
      </div>
    </div>
  </div>
</details>

<div class="d-sm-none" style="height: 600px"><!-- min height for < sm --></div>
<div class="d-none d-sm-block" style="height: 300px"><!-- min height for > sm --></div>

Divider

The Select Menu's list can be divided into multiple parts by adding a .SelectMenu-divider.

<details class="details-reset details-overlay" open>
  <summary class="btn" aria-haspopup="true">
    Choose an item
  </summary>
  <div class="SelectMenu">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Title</h3>
        <button class="SelectMenu-closeButton" type="button">
          <!-- <%= octicon "x" %> -->
          <svg width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
            />
          </svg>
        </button>
      </header>
      <div class="SelectMenu-list">
        <button class="SelectMenu-item" role="menuitem">Item 1</button>
        <button class="SelectMenu-item" role="menuitem">Item 2</button>
        <div class="SelectMenu-divider">More options</div>
        <button class="SelectMenu-item" role="menuitem">Item 3</button>
        <button class="SelectMenu-item" role="menuitem">Item 4</button>
        <button class="SelectMenu-item" role="menuitem">Item 5</button>
      </div>
    </div>
  </div>
</details>

<div class="d-sm-none" style="height: 600px"><!-- min height for < sm --></div>
<div class="d-none d-sm-block" style="height: 260px"><!-- min height for > sm --></div>

Additional filter and footer

If the list is expected to get long, consider adding a .SelectMenu-filter input. Be sure to also include the .SelectMenu--hasFilter modifier class. On mobile devices it will add a fixed height and anchor the Select Menu to the top of the screen. This makes sure the filter input stays at the same position while typing.

Also consider adding a .SelectMenu-footer at the bottom. It can be used for additional information, but can also greatly improve the scrolling performance because the list doesn't need to be repainted due to the rounded corners.

<details class="details-reset details-overlay" open>
  <summary class="btn" aria-haspopup="true">
    Choose an item
  </summary>
  <div class="SelectMenu SelectMenu--hasFilter">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Title</h3>
        <button class="SelectMenu-closeButton" type="button">
           <!-- <%= octicon "x" %> -->
          <svg width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
            />
          </svg>
        </button>
      </header>
      <form class="SelectMenu-filter">
        <input class="SelectMenu-input form-control" type="text" placeholder="Filter" aria-label="Filter">
      </form>
      <div class="SelectMenu-list">
        <button class="SelectMenu-item" role="menuitem">Item 1</a>
        <button class="SelectMenu-item" role="menuitem">Item 2</a>
        <button class="SelectMenu-item" role="menuitem">Item 3</a>
        <button class="SelectMenu-item" role="menuitem">Item 4</a>
        <button class="SelectMenu-item" role="menuitem">Item 5</a>
        <button class="SelectMenu-item" role="menuitem">Item 6</a>
        <button class="SelectMenu-item" role="menuitem">Item 7</a>
        <button class="SelectMenu-item" role="menuitem">Item 8</a>
        <button class="SelectMenu-item" role="menuitem">Item 9</a>
        <button class="SelectMenu-item" role="menuitem">Item 10</a>
        <button class="SelectMenu-item" role="menuitem">Item 11</a>
        <button class="SelectMenu-item" role="menuitem">Item 12</a>
        <button class="SelectMenu-item" role="menuitem">Item 13</a>
        <button class="SelectMenu-item" role="menuitem">Item 14</a>
        <button class="SelectMenu-item" role="menuitem">Item 15</a>
        <button class="SelectMenu-item" role="menuitem">Item 16</a>
        <button class="SelectMenu-item" role="menuitem">Item 17</a>
        <button class="SelectMenu-item" role="menuitem">Item 18</a>
        <button class="SelectMenu-item" role="menuitem">Item 19</a>
        <button class="SelectMenu-item" role="menuitem">Item 20</a>
        <button class="SelectMenu-item" role="menuitem">Item 21</a>
        <button class="SelectMenu-item" role="menuitem">Item 22</a>
        <button class="SelectMenu-item" role="menuitem">Item 23</a>
        <button class="SelectMenu-item" role="menuitem">Item 24</a>
        <button class="SelectMenu-item" role="menuitem">Item 25</a>
      </div>
      <footer class="SelectMenu-footer">Showing 25 of 25</footer>
    </div>
  </div>
</details>

<div class="d-sm-none"         style="height: 600px"> <!-- min height for < sm --> </div>
<div class="d-none d-sm-block" style="height: 380px"> <!-- min height for > sm --> </div>

Tabs

Sometimes you need two or more lists of items in your Select Menu, e.g. branches and tags. Select Menu lists can be tabbed with the addition of .SelectMenu-tabs above the menu.

<details class="details-reset details-overlay" open>
  <summary class="btn" aria-haspopup="true">
    Choose an item
  </summary>
  <div class="SelectMenu SelectMenu--hasFilter">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Title</h3>
        <button class="SelectMenu-closeButton" type="button">
          <!-- <%= octicon "x" %> -->
          <svg width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
            />
          </svg>
        </button>
      </header>
      <form class="SelectMenu-filter">
        <input class="SelectMenu-input form-control" type="text" placeholder="Filter" aria-label="Filter" />
      </form>
      <nav class="SelectMenu-tabs">
        <button class="SelectMenu-tab" aria-selected="true">Branches</button>
        <button class="SelectMenu-tab">Tags</button>
      </nav>
      <div class="SelectMenu-list">
        <button class="SelectMenu-item" role="menuitem">Branch 1</button>
        <button class="SelectMenu-item" role="menuitem">Branch 2</button>
        <button class="SelectMenu-item" role="menuitem">Branch 3</button>
        <button class="SelectMenu-item" role="menuitem">Branch 4</button>
        <button class="SelectMenu-item" role="menuitem">Branch 5</button>
      </div>
      <div class="SelectMenu-list" hidden>
        <button class="SelectMenu-item" role="menuitem">Tag 1</button>
        <button class="SelectMenu-item" role="menuitem">Tag 2</button>
        <button class="SelectMenu-item" role="menuitem">Tag 3</button>
      </div>
      <footer class="SelectMenu-footer">Showing 5 of 5</footer>
    </div>
  </div>
</details>

<div class="d-sm-none" style="height: 600px"><!-- min height for < sm --></div>
<div class="d-none d-sm-block" style="height: 380px"><!-- min height for > sm --></div>

Message

A SelectMenu-message can be used to show different kind of messages to a user. Use utility classes to further style the message.

<details class="details-reset details-overlay" open>
  <summary class="btn" aria-haspopup="true">
    Choose an item
  </summary>
  <div class="SelectMenu">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Title</h3>
        <button class="SelectMenu-closeButton" type="button">
          <!-- <%= octicon "x" %> -->
          <svg width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
            />
          </svg>
        </button>
      </header>
      <div class="SelectMenu-list">
        <button class="SelectMenu-item" role="menuitem">Item 1</button>
        <button class="SelectMenu-item" role="menuitem">Item 2</button>
        <div class="SelectMenu-message border-bottom border-top bg-red-0 text-red p-2">Message goes here</div>
        <button class="SelectMenu-item" role="menuitem">Item 3</button>
      </div>
    </div>
  </div>
</details>

<div class="d-sm-none" style="height: 600px"><!-- min height for < sm --></div>
<div class="d-none d-sm-block" style="height: 200px"><!-- min height for > sm --></div>

Loading

When fetching large lists, consider showing a .SelectMenu-loading animation.

<details class="details-reset details-overlay" open>
  <summary class="btn" aria-haspopup="true">
    Choose an item
  </summary>
  <div class="SelectMenu SelectMenu--hasFilter">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Title</h3>
        <button class="SelectMenu-closeButton" type="button">
          <!-- <%= octicon "x" %> -->
          <svg width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
            />
          </svg>
        </button>
      </header>
      <form class="SelectMenu-filter">
        <input class="SelectMenu-input form-control" type="text" placeholder="Filter" aria-label="Filter" />
      </form>
      <div class="SelectMenu-list">
        <div class="SelectMenu-loading">
          <!-- <%= octicon "octoface", class: "anim-pulse", width: 32 %> -->
          <svg
            width="32 "
            height="32"
            viewBox="0 0 16 16"
            class="octicon octicon-octoface anim-pulse"
            aria-hidden="true"
          >
            <path
              fill-rule="evenodd"
              d="M14.7 5.34c.13-.32.55-1.59-.13-3.31 0 0-1.05-.33-3.44 1.3-1-.28-2.07-.32-3.13-.32s-2.13.04-3.13.32c-2.39-1.64-3.44-1.3-3.44-1.3-.68 1.72-.26 2.99-.13 3.31C.49 6.21 0 7.33 0 8.69 0 13.84 3.33 15 7.98 15S16 13.84 16 8.69c0-1.36-.49-2.48-1.3-3.35zM8 14.02c-3.3 0-5.98-.15-5.98-3.35 0-.76.38-1.48 1.02-2.07 1.07-.98 2.9-.46 4.96-.46 2.07 0 3.88-.52 4.96.46.65.59 1.02 1.3 1.02 2.07 0 3.19-2.68 3.35-5.98 3.35zM5.49 9.01c-.66 0-1.2.8-1.2 1.78s.54 1.79 1.2 1.79c.66 0 1.2-.8 1.2-1.79s-.54-1.78-1.2-1.78zm5.02 0c-.66 0-1.2.79-1.2 1.78s.54 1.79 1.2 1.79c.66 0 1.2-.8 1.2-1.79s-.53-1.78-1.2-1.78z"
            />
          </svg>
        </div>
      </div>
      <footer class="SelectMenu-footer">Loading...</footer>
    </div>
  </div>
</details>

<div class="d-sm-none" style="height: 600px"><!-- min height for < sm --></div>
<div class="d-none d-sm-block" style="height: 220px"><!-- min height for > sm --></div>

Blankslate

Sometimes a Select Menu needs to communicate a "blank slate" where there's no content in the menu's list. Usually these include a clear call to action to add said content to the list. Swap out the contents of a .SelectMenu-list with a .SelectMenu-blankslate and customize its contents as needed.

<details class="details-reset details-overlay" open>
  <summary class="btn" aria-haspopup="true">
    Choose an item
  </summary>
  <div class="SelectMenu">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Title</h3>
        <button class="SelectMenu-closeButton" type="button">
          <!-- <%= octicon "x" %> -->
          <svg width="12" height="16" viewBox="0 0 12 16" class="octicon octicon-x" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M7.48 8l3.75 3.75-1.48 1.48L6 9.48l-3.75 3.75-1.48-1.48L4.52 8 .77 4.25l1.48-1.48L6 6.52l3.75-3.75 1.48 1.48L7.48 8z"
            />
          </svg>
        </button>
      </header>
      <div class="SelectMenu-list">
        <div class="SelectMenu-blankslate">
          <!-- <%= octicon "repo", class: "color-gray-3", width: 24 %> -->
          <svg width="24 " height="32" viewBox="0 0 12 16" class="octicon octicon-repo color-gray-3" aria-hidden="true">
            <path
              fill-rule="evenodd"
              d="M4 9H3V8h1v1zm0-3H3v1h1V6zm0-2H3v1h1V4zm0-2H3v1h1V2zm8-1v12c0 .55-.45 1-1 1H6v2l-1.5-1.5L3 16v-2H1c-.55 0-1-.45-1-1V1c0-.55.45-1 1-1h10c.55 0 1 .45 1 1zm-1 10H1v2h2v-1h3v1h5v-2zm0-10H2v9h9V1z"
            />
          </svg>
          <h4 class="my-2">No repositories</h4>
          <p class="mb-3 text-gray">We didn’t find any matching repositories that you can commit to.</p>
          <button type="button" class="btn btn-sm btn-primary">Create a repository</button>
        </div>
      </div>
    </div>
  </div>
</details>

<div class="d-sm-none" style="height: 600px"><!-- min height for < sm --></div>
<div class="d-none d-sm-block" style="height: 260px"><!-- min height for > sm --></div>

github.com usage

When adding the .SelectMenu component on github.com, use the <details-menu> element. It will 🎩 magically make the .SelectMenu work. Here a basic example:

<details class="details-reset details-overlay" id="my-select-menu">
  <summary class="btn" title="Pick an item">
    <span>Choose</span>
    <span class="dropdown-caret"></span>
  </summary>
  <details-menu class="SelectMenu" role="menu">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <span class="SelectMenu-title">My Select Menu</span>
        <button class="SelectMenu-closeButton" type="button" data-toggle-for="my-select-menu">
          <%= octicon("x", "aria-label": "Close menu") %>
        </button>
      </header>
      <div class="SelectMenu-list">
        <a class="SelectMenu-item" href="" role="menuitem">Item 1</a>
        <a class="SelectMenu-item" href="" role="menuitem">Item 2</a>
        <a class="SelectMenu-item" href="" role="menuitem">Item 3</a>
      </div>
    </div>
  </details-menu>
</details>

If loading content should be deferred, use the <include-fragment> element:

<details-menu class="SelectMenu" src="/my-menu" preload>
  <div class="SelectMenu-modal">
    <include-fragment class="SelectMenu-loading" aria-label="Menu is loading">
      <%= octicon("octoface", class: "anim-pulse", :height => 32) %>
    </include-fragment>
  </div>
</details-menu>

It will add a pulsing :octoface: icon while the content is loading.