Standard asset question

Great question, I am looking for a solution to that issue as well.

Maybe this could help https://answers.unity.com/questions/890758/how-to-move-an-image-just-in-the-surface-of-the-ca.html

Dunno how the movement is handled but the logic may be the same.

Well, I doubt many people on this forum want to hunt down the script that controls / clamps movement of that virtual joystick or get it hooked up to answer this question.

Is the joystick a UI element, controlled by scripts to stay on the screen? The immediate and trivial solution would just be to offset its clamp by its pixel dimensions.