Scroll up or down, it always happens on Chrome and not on FF.
Tested this multiple times even after clearing cache/cookie/browsing history in Chrome. It reproduces.
I started testing by disabling the Add on at which position this quirk happened. @AndyB's Inactive Member's add-on was the first (as that's what was visible first in the list where this happened as in screen shot above) to play with. Disabled the same and the quirk disappeared. Re-enabled again and the issue never reproduced even on Chrome.
So, this may pl be closed as not an issue.
Thanks for listening!