Re-measurement of the height of visible threads happened in an effect
that depended only on the set of top-level threads, not the subset that
were currently visible.

On desktop, hover events would cause annotations to be focused as the
user scrolled through the list, resulting in the root thread being
recalculated regularly and hence the internal `threadHeights` state in
`ThreadList` was usually accurate.

On mobile however, hover events don't fire and therefore `threadHeights` was
not updated as often as it should be. The mismatch between the actual
rendered height of threads and the value in `threadHeights` caused the
thread list to jump as a thread above the viewport was transitioned from a
rendered to a non-rendered state. This issue can be reproduced in Chrome
on desktop when the device type is set to "Mobile (Touch)" in dev tools.

This PR fixes the issue by making recalculation depend on the set of
_visible_ threads. In practice `visibleThreads` changes on most renders,
however the amount of work the effect does if no measured heights
changed is small, so this shouldn't be an issue.

In addition to fixing this issue, this commit also adds a test for the
overall behavior as the user scrolls through the list. Since correct
behavior depends on complex interaction between the DOM, `ThreadList`
and logic in two utility modules, we avoid mocking either the DOM or
utilities in these tests.