I am so glad this topic was posted! I recently had words with a passenger in a very similar situation as the OP, but it wasn't OP - there was no mention of an injury. This was domestic first flight (the kind where, as others mention above, there is about a 5" wide gap on the aisle, then a spot in the middle for a personal item, then a spot on the aisle for a personal item. I was in 2E and other passenger was 2F. I get on, and passenger has a bag in his compartment and his legs stretched in the middle (my) compartment. I place my carry-on above, sit down and get organized, and I start the conversation kindly, since there's no way I anticipate an issue:
Me, starting to bend over to put my laptop bag in my compartment: Sorry, I'm just going to put this under the seat.
2F: My feet are here, that's your compartment (pointing at the 5" gap).
Me: Well, that area where your bag is yours (pointing to where his bag actually is), this area is for mine.
2F, again pointing at the 5" space on the aisle: Then what's that space for? (This is where he gets red and starts panting in a little hissy fit)
Me: I have no idea but it doesn't fit any personal items. (Short and to the point, I'm not going to entertain this child... I'm about get the FA)
2F: Oh, so you make the rules now?
Me: Do you want me to get the flight attendant so she can explain how things go in the front cabin?
2F: I'd prefer you not to speak to me anymore.
Me: I'd prefer you to _________.
This all took place in the first 45 seconds of preboarding, and honestly we didn't have any issue the entire flight or in deplaning upon arrival, even shared the cup tray in the middle just fine. Maybe he was having a terrible day. I shouldn't have told him what to do at the end using those words. But in all my flying (my status is earned by flight volume, not by high spend on longhauls) I've never once had this happen. I split my F seats maybe 60% window, 40% aisle, and even in the window it's never crossed my mind to be offended with the aisle seatmate places their stuff in the middle -- there is literally no other place to put it, it's definitely for the aisle passenger.