VK’s Channel has a video called ‘How to Fix Your Joy-Con Drift Permanently’, which suggests that pushing down on the plastic around the stick on the left Joy-Con immediately stops the stick from drifting.
The need, then, is to find a way to permanently put pressure on this area to make sure drift doesn’t happen.
According to the video (which can be seen above and starts the tutorial at 5:55), the solution is to open up the Joy-Con and add a tiny piece of card around 1mm thick underneath the stick’s metal enclosure.
The explanation given in the video is that over time the metal enclosure holding the stick becomes loose and separates from the stick, separating it from its contacts.
