That's a reasonable "requirement", but
not universally agreed upon.
Some people didn't mind sitting with a ton of bricks on their lap and a
cord plugged into the wall.
I once had a PDP11/05 CPU box (the 5.25" version) on my lap while I was
troubeshootin soem part of the hardware (I forget what). On the grounds
that the end of this troubelshooting it was running programs and is
clearly a comnputer, does that make the 11/05 the 'first laptop'?
-tony