On some browsers like Firefox and Safari, with a range slider with the right handle selected if you click to the left of the left handle the right handle will not move. After that the left handle will be free to move anywhere even past the right handle. I tracked down the error to the translateRange method. The axis parameter is missing on the second call to translateValue. So this line:

value = this.translateValue(this.value(0,axis) + this.oneStep(axis));

Should read

value = this.translateValue(this.value(0,axis) + this.oneStep(axis), axis);