Drift is an analog stick reporting movement while nobody touches it. Why sticks wear out, how to measure the resting offset against a game's deadzone, and which fixes last.
Most analog sticks measure their position with two potentiometers, one per axis: a wiper sliding across a resistive track. Each movement wears the track a little, and dust working its way in through the opening around the stick makes it worse. Over time the reading at rest stops landing on true center.
The stick has not moved; its idea of where center is has. The controller reports a small, constant push in one direction, and anything in a game that responds to that push moves on its own.
No stick rests at exactly zero, so every game ignores readings near the center. That ignored region is the deadzone, commonly somewhere around 10 to 25 percent of the stick's travel. A stick can be drifting slightly for months while the deadzone hides it, then cross the edge one day and start turning your camera.
It also explains why drift shows up in one game and not another: they use different deadzones. A competitive shooter with a tight deadzone reveals drift that a game with a generous one never shows.
Connect the controller, open the Gamepad Tester and press any button so the browser can see it. The page shows the stick values as the controller reports them, with no game's deadzone in between.
Let go of both sticks and watch the drift check. Each stick is plotted against a dashed circle marking a typical deadzone, with its resting offset as a number from 0 (perfect center) to 1 (pushed all the way). Under about 0.05 is clean. Between that and roughly 0.12 is wear that most games still ignore. A stick that rests outside the circle will move things on its own.
Then push each stick to the edge in all four directions and release it, watching where it settles each time. A healthy stick returns to the same spot. One that settles in a different place depending on which way it was pushed has a worn centering spring or track, and that inconsistency usually gets worse before anything else does.
Controllers with Hall effect or TMR sticks measure position magnetically, so there is no resistive track to wear out. They cost a little more and are the only fix that addresses the cause rather than the symptom. Whichever you buy, a minute with the tester before the return window closes is worth it: a stick that rests off-center out of the box will not improve.