Fix event handling in a Shadow DOM

[sbaechler]

What:

This fix adds a traversal through shadow roots to fix a long standing issue that Downshift is not properly working when rendered inside of a shadow DOM.

Fixes #1622

Why:

useMouseAndTouchTracker adds a bunch of event listeners to document in order to tell if the Select is open or not, but event.target represents the last <div> in the light DOM, rather than the <li> within the shadow DOM. Thus, targetWithinDownshift always fails, and the Select is closed before the button's onClick is able to be called.

How:

The utility function targetWithinDownshift now iterates through shadow roots to find the real active element.

I also added a Cypress test to reproduce the issue in (/shadow-dom/useMultipleCombobox) as well as a few unittests.

We did have a patch for version 6 that also patched the tab order. However, in my manual tests in version 9 I did not see any issues with the tab order anymore.

Checklist:

• Documentation N/A
• Tests
• TypeScript Types N/A
• Flow Types N/A
• Ready to be merged
  

I could not get the pre-commit hooks to pick up the proper test and format configuration. It might need some reformatting.