My wife, our infant, and I boarded with our carry-ons and personal items. Before boarding, the gate agents announced they were looking for volunteers to check bags because overhead bin space would be limited.
To help, we kept both of our personal items under the seats in front of us. The only thing we placed in the overhead bin was our diaper bag since there wasn't enough room at our feet.
About halfway through boarding, another passenger started opening closed overhead bins, shifting people's bags around, and trying to create space for her luggage. She eventually pulled our diaper bag out, handed it to us, and said, "Can you take this? I need this space."
My wife pointed to the area under our seats and explained that we didn't have room because of the baby and our personal items. Instead of asking or waiting for assistance, the passenger moved our diaper bag into another overhead compartment herself, then squeezed her suitcase into the spot where our bag had been. She even pushed our diaper bag farther into the other compartment so she could fit an additional tote bag overhead instead of keeping it at her feet.
As soon as we landed, she was one of the first passengers up to collect her belongings. When we retrieved our diaper bag, we discovered one of the baby bottles had leaked, most likely because the bag had been shoved around so forcefully.
The whole interaction was incredibly frustrating. If it had only been my wife and me, I probably would have confronted her. With our baby there, though, I chose to avoid escalating the situation, even if it meant quietly dealing with the frustration.