This issue tracks the progress in implementing IntersectionObserver API (https://w3c.github.io/IntersectionObserver/). Each steps in this issue has a corresponding TODO comment in the code.

Initial Support

• dom: Stub out the IntersectionObserver interface #33989
• dom: IntersectionObserver initialization #35314
• dom: Implement minimal IntersectionObserver workflow #35551

Steps

• Consider CSS transform and scroll offsets in element get bounding box.
• Query for containing block of a node in a fragment tree and mapping of coordinate space for step 2.2, step 4, and step 6 in compute intersection algo.
• Overflow content-clip clipping for compute intersection algo and root intersection rectangle.
• Recheck scrollbar related clipping for compute intersection algo and root intersection rectangle with Servo's approach to scrollbar.
• Several security cases of cross-origin-domain target.
• Implement Document's pending initial intersectionobserver targets, an additional condition in HTML Event loop.
• The time attribute should be relative to the time origin of the global object. #dom-intersectionobserverentryinit-time
• Handle query whether a node is descendant of other node for step 6 of 3.2.10 run update the observation intersection steps
• Inter-document coordinate mapping and bounding rectangle queries.

Additional Steps

• Clip element according to it's clip-path for compute intersection algo and root intersection rectangle.
• Skip further processing if the element is hidden in update intersection observation algo.
• For a scrollport apply the scroll margin.
• Recheck isIntersecting definition according to the discussion.
• Implement 3.2.8 Calculate visibility algo & 3.2.9 Effective transformation matrix.
• Tidy up IntersectionObserverRootMargin #35907
• Implement lazy load intersection observer.