Generally, I think it is poor form to lean on a car while at rest in traffic. It happens and is nothing to have a cow about, but it is poor form.
One may wince when hearing of motorists who are slightly precious about their cars, but this is little different (if at all) from the attitude of some cyclists who appear to think that their steed is somehow special because it cost a lot. At a guess, I think I know more cyclists who'd be uppity about someone touching or leaning on their bicycle than I do motorists who think that way about their car. Most cars are left in the street all night. Most bicycles are not. Many top-end bicycles are treated and spoken of almost as religious icons.... Tape and frame must match... Bidon must match saddle... But I digess... as usual.
I giggle when I read the "If you did that to my car you'd be on the floor" comments on these threads. They tend to be from male posters (I nearly wrote male members) and if they were even remotely close to reality, there would be a lot of cyclists being banged on the noggin and sent floorwards with venom. There are not. There is a huge difference between sounding tough on the Internet and an act of road rage... Thank Goodness.
I confess to using the rear pole of Routemasters when a teen and pre-teen, but even then the conductor would always try to shoo away cyclists and sateboarders. I knew it was naughty and that was part of the fun.
I've never used a car, but did sprawl onto the bonnet of a taxi on Poland Street once in a clipless moment. I apologised and the driver just giggled and waved the apology away.
Done deliberately it is poor form, but more a case of thoughtless ignorance of the presence and concerns of others than a hanging offence.
We should all be lovely and smile more.
Thank you.