Indeterminate Progress Bar

A server component, there are no hooks. The track is marked role=progressbar with aria-busy and without aria-valuenow — that is the correct way to declare indeterminate progress; an invented percentage is worse than none. The beam travels along the track with a transform animation rather than by changing left or width: the browser does not recalculate layout on every frame, and the bar does not heat the CPU inside a long list. The thickness is exposed through the height prop and fed into a variable in pixels. Under prefers-reduced-motion the beam fills the whole track and breathes slowly. The theme comes from the surrounding color-scheme through light-dark(): the component has no background by default — it sits on the page background and darkens with it, while the background prop switches on a panel.