If an inviter leaves a 1:1 before the invitee joins, we should cancel the invite

or somehow better handle the “you cannot join an empty room” error

Begin by locating the Matrix specification section covering 1:1 invitations and room joins, then compare the inviter-leaves case with the empty-room error described here. Done means the specification defines one unambiguous outcome—cancelling the invite or handling the error—and documents the corresponding behavior.

Written by the indexing model from the issue text.