Electronic – Mains voltage waveform on oscilloscope

I guess the USB shield isn't grounded. Strange but not impossible.

Without a ground reference, the scope will measure the difference between its own ground or chassis, and the laptop ground. If the laptop has only a two pin plug, then its ground will be floating at about VSupply/2 (RMS) because of the capacitors in its EMI filter.

Clip the scope ground to the Arduino ground to force the laptop ground to the same as mains ground, and the scope will start to read more normally. A small current will flow, it's limited by law to perhaps 200 uA, but might be a lot less.

In general on an experimental bench, you should connect the equipment ground to mains earth. The 100 V at 100 uA won't hurt you, and your laptop will be protected against it, but the Arduino isn't, and you can easily blow things by touching them.

The alternative is to buy a cheap differential probe. I had bought one on Ali- Micsig DP10013 and it works surprisingly well. However a handheld scope should have a triax connector and differential input. Many cheap handheld DSO don't have this option, there might be some lurky datasheet that you won't understand at first glance, but then it's too late. Go visit eevblog site for opinions about the DSO you want to buy.

So if the handheld scope is not accessible, then go for HV differential probe.