How can I get e.offsetX on mobile/iPad

You can use clientX or pageX, see here

The correct answer based on the comments in the suggested answer:

e.offsetX = e.touches[0].pageX - e.touches[0].target.offsetLeft;     
e.offsetY = e.touches[0].pageY - e.touches[0].target.offsetTop;

This ignores any transformations such as rotations or scaling. Also be sure to check if there are any touches.

Thanks, @Kontiki - this is the solution that finally fixed things for me:

if("touchmove" == e.type)
{

    let r = canvas.getBoundingClientRect();
    currX = e.touches[0].clientX - r.left;
    currY = e.touches[0].clientY - r.top;
}
else
{
    currX = e.offsetX;
    currY = e.offsetY;
}