Keep floating placed against anchor while it is shown, on engines
without CSS anchor positioning. Positions immediately, then again on every
scroll (anywhere in the document) and window resize, so a surface opened
from a scrolling container follows its anchor instead of drifting. Returns
a disposer that stops listening. On engines with anchor positioning it only
stamps the requested side onto data-hl-side (so the stylesheet can pick a
position-area) and returns noop, so callers can invoke it unconditionally.
Keep
floatingplaced againstanchorwhile it is shown, on engines without CSS anchor positioning. Positions immediately, then again on every scroll (anywhere in the document) and window resize, so a surface opened from a scrolling container follows its anchor instead of drifting. Returns a disposer that stops listening. On engines with anchor positioning it only stamps the requested side ontodata-hl-side(so the stylesheet can pick aposition-area) and returnsnoop, so callers can invoke it unconditionally.