As discussed in immersive-web/webxr-polyfill#116 (comment) calling xrSession.cancelAnimationFrame from within a frame callback should be able to cancel another subsequent callback within that same frame's callbacks list. A strict reading of the current algorithm as written does not allow for this; it says to search through the "list of animation frame callbacks" for the handle to cancel, but that "list of animation frame callbacks" is explicitly set to an empty list prior to callback execution.