The shouting is feminine and angry and, Steve realizes as he gets closer, absolutely coming from Bucky's open door. By the time he's close enough to make out the words, half the building's got their windows or their doors open to listen. A good fight is always entertainment, especially when it's happening out of doors.
Katy comes out the door first, stamping down two steps and then turning to throw her coat in Bucky's face when he follows her out onto the landing without his shirt and with an extremely confused expression. He catches the coat, bewildered, and then Katy snatches it back out of his hand.
Steve elects, on balance, to stop where he is and lean on a corner until she's finished. Katherine Mulligan's not above involving bystanders in her fights, and she's got a mean kick even in heels.
She stops at the bottom of the stairs, turns and stabs a threatening finger at Bucky and says, "And someday you're going to fall in love, you absolute bastard, and she's going to ruin you and they'll find you drinking your heart drowned and singing sad songs and crying and I just hope I'm there to see it and may your soul rot, James Buchanan Barnes, may it rot and I don't even care you're going off to France to get killed because I hate you!" and then she turns on her heel and storms right in Steve's direction.
He steps prudently out of the way, but she stops near him anyway and stabs a finger at him this time, her bag swinging at her hip, and then back at Bucky again and says, "And you're welcome to the wretch! Welcome!" and then finishes her storming.
Her clothing, Steve notices, looks like she put it on pretty quickly: she's got a run in her stockings and everything's a little bit crooked and her hair's a bit mussed. After watching her go for a moment, he gets back on his way.
Bucky's still leaning with his forearms on the railing, shirtless and staring after her with a frown of the purest bafflement possible. "What did you do?" Steve asks, walking up the steps she'd just stalked down.
"I honestly don't have the faintest fucking idea," Bucky says. "I seriously don't know." He runs a hand over his hair, rubbing his neck in puzzlement and adds, "I guess we're over. She took all her things. Fuck, I don't know. Maybe she'll tell her brother and I can get it out of him after he tries to be her avenging angel."
He looks so confused - and so hung-over - that Steve works hard not to laugh at him, and pats his shoulder instead. "Good job," he says. "You should probably put a shirt on, buddy."
Bucky glances down at himself, as if he hadn't noticed he was half naked, and puts a hand over his face. "Ah fuck," he says, and then glares at everyone still watching, "alright show's over the lot of you, no more entertainment for today," and stomps back into the apartment. Steve shakes his head and follows.