Simple Player Online Detector Minecraft 1.16+
Ever since stray cats were added to Minecraft it is possible to detect whether the owner of a cat is online and in the same world. By using an ancient mechanic of cats sitting on top of chests and blocking them, a comparator can be used to detect that the cat sat down or stood up. When a player logs out or switches worlds, the comparator will update and will output whether the player is in the same world.
In many minecraft versions there is a bug that leashes can break under rare circumstances. If you have problems with this detector in some versions, build it inside the spawnchunks.
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