Yeah, I’d maybe make a set of just the position IDs, and do a negated .has() of that set to check if the cursor was removed.

    ws.onmessage = function(event) {
      const positionIds = new Set(positions.map(pos => pos.id));

      Object.keys(cursors).forEach((id) => {
        if (!positionIds.has(id)) {
            document.body.removeChild(cursors[id]);
          delete cursors[id];
        }
      });
    };

Untested but that’s probably pretty close for phone typing