Fenced frames or URN iframes, if loaded through an API like Protected Audience or Shared Storage, can send out reporting beacons automatically if some event occurs (currently only top-level navigation beacons are supported). We previously tweaked this feature to allow cross-origin documents loaded in the root fenced frame's tree to send automatic beacons if opted in, but still kept the restriction that only frames that are same-origin to the origin loaded by the API could set the data that would be sent as part of the beacon. This feature expands that functionality to allow the cross-origin document to set the data that will be used in the automatic beacon. To allow this while still preserving privacy, both the fenced frame root document and the cross-origin subframe document must explicitly opt in. This is the same opt in shape as other cross-origin FFAR features. Specifically, the root frame must opt in via the "Allow-Fenced-Frame-Automatic-Beacons" header, and the cross-origin subframe setting the data must opt in via the 'crossOriginExposed' parameter in the call to setReportEvent...().
Fenced frames or URN iframes, if loaded through an API like Protected Audience or Shared Storage, can send out reporting beacons automatically if some event occurs (currently only top-level navigation beacons are supported). We previously tweaked this feature to allow cross-origin documents loaded in the root fenced frame's tree to send automatic beacons if opted in, but still kept the restriction that only frames that are same-origin to the origin loaded by the API could set the data that would be sent as part of the beacon. The existing setup assumes that payload data will only ever come from the buyer directly. However, there are cases where a buyer embeds a cross-origin subpage that contains data that needs to be sent with an automatic beacon. This limitation forces the same-origin root ad document to be an intermediary between the page with the data and the automatic beacon API, causing unnecessary extra overhead and forcing extra data to be sent directly to the root fenced frame.
Explainers: https://github.com/WICG/turtledove/pull/1386