When moving the gimble horizontally in a straight line the camera  does  not stay locked but instead has a slight amount of rotation - drift.  I  have  tried shooting with lock trigger on and with disabling the smooth  track  but it makes no difference.  How can I do a horizontal pan with  the  camera being locked and not rotating?

I have rebalanced the gimle, done auto tune, recalibrated, disabled  smooth pan, tried holiding the lock trigger but nothing works

in 'Torchlight Mode' (near and in  horizontal operation, as in video 0:15" onwards) the gimbal has been programmed to avoid certain movement in order to avoid hardware crashes.
Unfortunately the programming of those is poor and leads to resulting problems, as you have indicated. I myself have brought this to DJI's attention multiple times previously, and still await eagerly a fix for this nasty issue, check this thread:https://forum.dji.com/forum.php?mod=viewthread&tid=170728#pid1871451
I hope your contribution has an outcome