Leaks from Otherworlds: Discovering Protocols for Spontaneous Mixed Reality Crossing
Mixed reality supports public art, play, and self-expression, yet people sharing a street may inhabit mutually invisible digital layers. Making a layer discoverable can reveal information or demand attention before anyone chooses to engage. We study \textbf{spontaneous crossing}: noticing an unfamiliar mixed reality and negotiating whether to take part. In an HCI classroom workshop with 40 participants we used \textbf{Protocol Futuring}, a speculative method in which teams propose the rules of an imagined encounter, challenge them, and revise. Comparing the questions this raised with existing literature, we map five facets: \textbf{leakage} and discoverability, which covers deliberate signals and incidental exposure alike; the crossing process; intentionality; Reality Access Control; and interoperability. Walkthroughs of a menu-based invitation and a handshake gesture show how one entry choice reshapes later permissions and responsibilities. This initial design space connects playful encounters to questions of who may see, join, refuse, and shape a shared reality, including those affected without taking part.