Why, it's our Maximum Leader, of course! She has right-of-way under all conditions.
You may use the stair when she is done with it and not before.
In our house, we have a very simple set of traffic laws. Everyone must make way for our Maximum Leader. She can sit anywhere, walk anywhere and lay anywhere. You must go around her. She's not pushy or demanding, she's just relaxed and happy. She knows we all love her and would never hurt her, so when she plops down on the floor or a chair or the stairs, she's there for the duration. She's almost always around, too. She's not a lap cat, but she really likes our company. She just likes to hang out wherever we are.