You should unplug your laptop when it finishes charging to avoid permanently damaging the battery.
Modern laptops (since approximately 2015) implement battery management controllers that cease charging once the battery reaches 100%, preventing overcharging and its associated stress. Even older systems with less sophisticated controls experience minimal degradation from float charging at full capacity; the larger threat to battery lifespan is repeated deep discharge cycles or heat. Keeping a laptop plugged in at room temperature produces negligible cumulative wear compared to portable use with frequent charge cycles. Some manufacturers (Apple, Lenovo) now offer 'Conservation Mode' to cap charging at 80% for users who primarily work plugged in, optimizing for extreme longevity, but this is optional, not required. The myth reflects concern about ancient lead-acid or nickel-cadmium batteries, where overcharging posed genuine chemical risks; lithium chemistry changed the equation entirely, yet the superstition persists.