A bit of story time first - I flew EY85 in Y recently, and boarded ahead of nearly everyone due to VA Gold. Y was almost completely full - all but 4 seats occupied - as such, the overhead bins were also chock full on the 77W. I put my large-ish backpack in the overhead bins, with my puffer jacket underneath the seat, and settled in.
One of the last few passengers to board was a group, potentially American-Italians (not sure, they spoke English only but with an accent), who all had roller bags and at least a jacket, or sometimes another bag as well - they were sitting in the row in front of me. The overhead bins were mostly full at this stage, so they started looking for other places to put their bags.
This is where it gets interesting - one of the ladies in the family decided to remove my backpack and place her bag in its place - the only reason I noticed was because it must have been heavier than expected (it had both my laptop and portable monitor in it, among other things), and she accidentally hit the person in the aisle seat with it (I was at the window). I asked her she was doing with my bag, and she said she needed to move it because her bags "had priority". I said I put it there for a reason, but she simply ignored me and put my bag about 3 rows down, making it an absolute pain to get to when deplaning.
I left it there then, as I was in the window seat and didn't want to cause any actual issues for the FA. But what would you guys have done? If she had asked, I probably would've been okay with moving my bag, and could've warned her it's heavy cause laptops etc. But the fact she just took it and moved it really didn't sit right with me - what happens if I didn't realise, or if she accidentally broke something? I'm sure there's some interesting AFF stories on people touching things that aren't theirs