We've done this thread a few times before (I know Search is down...).

But...I'm going to guess here...I think the OP was talking about a situation where the aircraft door has closed and the seat is still empty. Then she sprawls out a bit. It's a situation where no one has booked the seat, and she's claiming the territory since it's right next to her.

I have to admit: I do the same thing. If it's 3-wide seating, the other person (aisle or window) does as well. We use whatever space is there, use the tray table, etc. I don't see any problem with it.

Two cases where this would be extremely rude, selfish, inconsiderate, etc.:

(a) On Southwest, before the door closes, sprawling out to "guard" multiple seats. I see people doing this in the exit rows all the time, and I agree it's selfish. People hope that if the flight goes off with 1 empty seat, it'll be the one next to them. I say if you really are hopeful about getting an empty seat, head straight for the last row of the plane and don't take exit row.

(b) On any other airline if somebody actually has the seat assignment. Duh, Captain Obvious, it's rude to try and prevent someone from sitting there, but I've had it happen to me on occasion - where I can tell that the person would rather I headed to the back and found another seat. (I don't think this is what the OP is talking about.)

What works for me: if I really want an empty seat, I talk to the gate agent a few minutes before boarding. I usually can tell by looking around whether there's any chance at all - if it's a packed flight, then obviously I don't bother them. But...if it's a lightly loaded flight, I politely ask for it, making it clear that I'll take whatever they have. (In other words, don't go in acting like you want an exit-row seat block; be willing to take the last row if you really want an open seat.) That usually works well.