The [visibilitymaskchange](https://immersive-web.github.io/webxr/#eventdef-xrsession-visibilitymaskchange) event doesn't have an equivalent `onvisibilitymaskchange` event handler property defined in the [`XRSession`](https://immersive-web.github.io/webxr/#xrsession) WebIDL. This should be added.
The visibilitymaskchange event doesn't have an equivalent
onvisibilitymaskchangeevent handler property defined in theXRSessionWebIDL.This should be added.