Since the `<thread-list>` is now contained within a scrollable element
which is not `document.body`, it needs to scroll that element rather
than the window in order to scroll the element into view.

 * Remove the hard-coded selector used to find the scroll root and
   instead search for the closest ancestor element to the
   `<thread-list>` whose computed style has `overflowY` set to `scroll`.
 * Set the scroll root Element's `scrollTop` to scroll the thread list
   instead of calling `window.scroll`
 * Compute the maximum scroll offset based on the `scrollTop` and
   `scrollHeight` properties of the scroll root element instead of
   using `document.body.scrollHeight`.
 * Update tests for `<thread-list>` to check for the scrollable
   ancestor being scrolled instead of the window.