While looking at https://wlcg-traceability-isolation-wg.webtest.cern.ch/, I think there is a minor display glitch: the page starts getting rendered without the top “CERN Accelerating science” bar, which only appears a bit later. This result in a weird visual glitch, in which the menu appears then slides down to accommodate that bar. It would be nice if the bar was rendered from the very beginning.
(I didn’t try to debug or look at the code, so I might be wrong on the root cause)
I am not immediately able to reproduce this behaviour. It sounds weird though because there should be no differences between the live .web.cern.ch and the preview .webtest.cern.ch: Are you able to re-produce this behaviour consistently? Could you please try and clear your caches?
However, if this is something you can re-produce on the .web and .webtest sites, it is likely something which has been there for a long time. As a lot of things are happening right now, I suggest we re-visit this following the Drupal 9 upgrade. It might even resolve this issue! In any event, if you are still seeing this once on Drupal 9, please submit a bug report.