Focus trapping fails to account for elements within shadow DOM

We are building a web component that uses a11y-dialog—this is similar to the framework implementations of the library but for web components.

We’ve noticed that focusable elements within shadow DOM are not picked up by the logic in _maintainFocus. This is because event.target.closest is not able to see into shadow DOM.

I’m wondering if you’re open to a PR (and/or collaboration) that would upgrade this logic with shadow DOM support so this library can work properly with web components.

Thanks in advance!

[vilchesalves]

I was trying to use a11y-dialog with templates and slots, but was hit by this issue. I’m happy to see it here. I ended up reimplementing a lot of it, and in the meantime I wrote something that might help you as well.

/**
 * @param {string} selector
 * @param {DocumentFragment | Element} root
 * @returns {HTMLElement[]}
 */
export const piercingSelectorAll = (selector, root) => {
  const result = [];

  // matching the root against the selector
  if (root instanceof HTMLElement && root.matches(selector)) {
    result.push(root);
  }

  // matching the root's children against the selector
  const children = /** @type {HTMLElement[]} */ (
    Array.from(root.querySelectorAll(selector))
  );
  result.push(...children);

  // matching shadowRoot's children against the selector
  for (const element of result) {
    if (element.shadowRoot) {
      result.push(...piercingSelectorAll(selector, element.shadowRoot));
    }
  }

  return result;
};

    const slots = Array.from(this.querySelectorAll("slot"));

    const assignedElements = slots.flatMap((slot) =>
      slot.assignedElements({ flatten: true })
    );

    const selectableElements = Array.from(
      new Set(
        assignedElements.flatMap((element) =>
          piercingSelectorAll(focusableSelectors.join(","), element)
        )
      )
    );

So then the $$ function could be something like:

  function $$(selector, context) {
    return selectableElements.filter(element => element.matches(selector))
  }

I hope you find it useful. 😃

[elidupuis]

Great to hear!

Admittedly, this stuff is relatively new to me as well. I’ve done some initial research and found https://github.com/salesforce/kagekiri, which provides “Shadow DOM-piercing query APIs”. At this point, I’m assuming we could pull the relevant logic out of that library or use it as a guide for implementing the specific methods we need in a11y-dialog. I think the element.closest() logic in _maintainFocus I mentioned above is only a couple of short methods but I’ve only done preliminary testing on that.

That said, I’m realizing that basically anywhere a11y-dialog currently uses querySelector (or similar APIs) likely doesn’t work with web components because it can’t pierce the shadow DOM. I’ll do a bit more digging to assess the scope of this and share my findings—hopefully within a couple of days.