Some devices will notice when they've been plugged in for a very long time, and will automatically discharge the battery to 80% or so, but continue to report 100% state of charge to the user.

Chemically speaking, Li batteries do degrade much faster when kept at 100% for weeks on end compared to being stored at 50%, but not as fast as cycling it. The absolute worst case (without cycling) is being stored at 100% at warmer temperatures, like 40C. They will lose over 10% within a month in those conditions. I highly suggest a laptop cooler for any desk where you may use a laptop plugged in for extended periods.