The OP can't really be at fault here when the other passenger was offered another seat but declined.
Same thing happened to me on a London Underground train, where the other passenger decided that he needed half my seat and armrest to work on his 17" computer. There he also could have had another seat but declined.
As this story goes, he eventually cocked back his elbow and elbowed me in the chest, after which (aided by the log on screen on his computer and his blog) I filed assault charges with the London Transport Police. Of course, he claimed it was my fault, so it wasn't dropped, but the police in closing the issue with me said he sure wasn't happy when he was taken down for questioning.
It's not about whinging, it's about respecting people's personal space.