Part of #3591 (mara: "pop ups and menus appearing should animate").
Each popover mounts fresh on open ({open ? <div> : null}, not a state
transition), so a keyframe animation on the popover element itself is
the right tool -- same shape as Shell.css's own shell-page-enter
(fade + a slight translate/scale settle), including the identical
three-rule motion-guard (base rule, prefers-reduced-motion media
query, data-motion=reduce/allow explicit overrides).
Scope: LinksMenu, SettingsMenu, UserMenu -- the three header popovers.
Not included here (posted findings on the issue instead of guessing):
the refresh-interval picker's dropdown (native <select>, whose open
popup is OS/browser chrome outside CSS reach in current browsers --
"not themed" is a platform limitation, not a bug in this component's
own styling) and the jobs graph's node animations (JobqGraph is a
@hive/shared component consumed by both swarm-ui and the per-hive
dashboard, real design/implementation work on shared infra, not a
same-shape mechanical extension of an existing pattern).
Screenshot-verified the settled (post-animation) state renders
correctly; a static screenshot cannot show an in-flight CSS animation,
so this leans on exact structural parity with the already-shipped
Shell.css pattern for the animation's own correctness.
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