Behaviour
- Show after: pixels (default 300) or a percent of the page
- Minimum scrollable pixels: hide everything when the page is shorter than this (default 80)
- When to show: always after the threshold, or only while scrolling up
- Motion: smooth or instant. Duration and easing apply to smooth. Reduced motion always jumps instantly
- Scroll target: optional CSS selector. An invalid selector falls back to the top
- Sticky header: a pixel offset, or a selector whose height is measured on click
- Scroll container: leave empty for the window. Set a selector only if the page scrolls inside another element
- Move focus to the top: off by default