`<thread-list>` uses `getComputedStyle` to find the nearest scrollable
ancestor element which it needs to know in order to scroll specific
annotations into view and determine which annotations are visible.

In Firefox `getComputedStyle` may return `null` if the iframe is hidden
- which may be the case when this code is called in the sidebar.  This
is considered a bug upstream. Work around it in the meantime by adding a
specific class (`js-scrollable`) to the scrollable element and testing
for that as a fallback.

Fixes #341