Spinner, Progress & RadialProgress

Busy indicators, split by whether you know the fraction. Spinner is indeterminate — a small animated mark for work of unknown duration; Progress is a linear bar (determinate when you pass a value, indeterminate when you don’t); RadialProgress is a ring with the percentage in its centre:

Spinner()                                  // a bare mark + off-screen "Loading"
Spinner(kind = SpinnerType.Dots, tip = "Fetching…")

Progress(value = Some(70))                 // 70 %
Progress()                                 // indeterminate animation

RadialProgress(value = 33)                 // a ring showing "33%"

Spinner also has an overlay mode: give it children and it floats the mark over them, dimming and disabling the content while spinning, then reveals it when spinning = false:

Spinner(spinning = loading)(
  div(/* the panel being loaded */),
)

SpinnerType picks the animation (Spinner/Dots/Ring/Ball/Bars/Infinity), and Color/Size tint and scale it. Progress renders the native <progress> element, so assistive tech reads the value for free; RadialProgress exposes role="progressbar" with aria-valuenow, takes size/thickness overrides, and shows custom centre children (or showValue = false for a bare ring). All three take their look from the active skin. For a placeholder while content loads (rather than a busy indicator), see Skeleton.