If I had a dollar for the amount of times I was
#1 on the list, anticipating a battlefield UG, only to have the door close with me still in the back, well...
In those rare cases that I'm pretty certain I'll clear a battlefield UG after boarding, I'll just stow my bag over the bulkhead in Y if I'm not already sitting there. If/when I clear, I'll move the bag to empty space in F (if same exists) or leave it, able to retrieve it without much trouble on arrival.