It's very annoying and getting worse. I've had it happen to me in domestic first in the US. Seriously, he didn't have enough room in his own seat that he needed to lean over the wide armrest into mine? Or worse yet, spread and extend his legs so they are well into my space.

I've found that asking them to respect my space usually works. Once I had a particularly obnoxious person next to me and I told him I'd be happy to sell him some of my space, but I only take cash. He gave me a seriously dirty look but did get back into his own space.