The Timeline will vertically jump up-&-down as the loading indicator appears and disappears when complex queries are being processed.
To reproduce: Open https://tools.wmflabs.org/interaction-timeline/?wiki=enwiki&user=Kaldari&user=DannyH%20(WMF) and scroll down.
Video: https://drive.google.com/open?id=147dcYgrvISypou_FR4kbwvqkzv-B_gCQ
This may be solved in T184146: Sub-epic ⚡️ : Create an API service for InteractionTimeline, but opening to track.