GradientBorder

Sends an arc travelling around the ring. It needs the theme's `motion.tier` above `minimal` and at least one edge left in `edges`; failing either, the static gradient ring is what you get.

Composes with the classes `Box` computes instead of replacing them.

The element it paints.

Which sides the beam lights, clockwise from the top. Read only while `beam` runs — on a static ring the gradient always goes all the way round.

A theme gradient role, or a literal `{ from, to }` pair. With `beam` running it stops painting the ring — the ring drops to the plain border colour and the gradient moves into the travelling arc, which is the only way the arc is visible against it.

How the arcs are spaced in time. `"continuous"` chains them back to back over the edges that are lit, so the beam never breaks; `"spaced"` keeps every edge in its own quarter of the cycle, so dropping one leaves a gap rather than closing it up.

What fills the inside of the ring. `"none"` leaves it transparent, which is what keeps this a ring drawn around existing content rather than a card with a gradient edge.

Thickness of the ring, as a raw length.