DEC/Compaq/HP had working handheld systems running Linux in 2000, starting with the Itsy [1] and moving on to versions of the Compaq iPaq and HP Jornada 720. The kdrive X server was written for these machines. I was running the same software stack on a ruggedized tablet that we manufactured.

[1] https://en.wikipedia.org/wiki/Itsy_Pocket_Computer