Timeline travel problems when timestamps are identical

[stonier]

Reproducible with the following code in viewer.py:

def send_tree(web_view_page, demo_trees, unused_checked):
    if send_tree.index != 1:
        demo_trees[send_tree.index]['timestamp'] = time.time()
    else:
        demo_trees[send_tree.index]['timestamp'] = demo_trees[send_tree.index-1]['timestamp']

• When only two trees, with identical timestamps, are in the cache, they slam up together on the edge of the timeline
• Get locked in a loop selecting itself as next (or vice versa with previous)
• Maybe incidental, highlight of the event marker sometimes gets lost