As a user, when I scroll down the page, if my mousecursor is over the vertical strip that contains the action menus, then each one will rapidly appear and disappear.
An editor suggests adding a 100ms delay (or similar), to fix this minor annoyance, at https://www.mediawiki.org/wiki/Topic:Sakwlbwmplal8umy
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Event Timeline
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As Matt says, we could even implement this in CSS (for the benefit of and consistency with non-JS mode), perhaps using animation-delay or something.