General tip:
If you have to pull out from the gutter at any point – taking the lane, changing lanes, or being in a different lane – make eye contact. Helps loads.
I get that you qualified that as a general tip but as the guy was holding his bars at the time it would have been difficult to maintain eye contact!
OP – well done for not spilling, I’m impressive with your stability, I reckon I’d have ended up on the road.
I’d imagine he either had someone in the car he was showing off too – or had a fight with someone before driving.
However, if you see him again be sure to publicise his details, then everyone that sees the car can check the wing mirrors for stability as we never know when we might have to use them.