It seems to be caused entirely by `scroll-behavior: smooth;` on the `html` element.
Try add that style to this (HN) comments page. Same problem.
Seems like the vim addons aren't dealing with browsers' implementations of that style. Whoops.
For the addon I use (Vimium-FF), it appears to have been already reported and fixed: https://github.com/philc/vimium/pull/3111 but not yet published to AMO.