We're getting XR_ERROR_SESSION_NOT_RUNNING sometimes when trying to start WebXR sessions. The problem is that there is nothing that prevents the render loop to call xrWaitFrame() and the rest of the methods before the session is actually started. WebXR code can properly setup the frame update callback before the OpenXR session begins, and thus the render loop would try to call xrWaitFrame()->xrBeginFrame()->xrEndFrame() on a session that hasn't started yet.