If you want to see the distorted current through the light bulb, put a
1 ohm resistor between the "ground" side of the light bulb and the
ground lead. Then put the oscilloscope probe on the hot side of the 1
ohm resistor. That way you do not need an A-B scope input.

Just make sure the "ground" side of the light fixture and the ground
through the scope probe of the scope and the ground of the chassis of
the oscilloscope are all at the same potential before you do this.

H. R.(Bob) Hofmann