Henyoku no Drops

The year is 3015, the ocean covers 90% of the planet, and it is normal for humans to fly in the sky. People who cannot fly are called "Drops" (the old generation), and Tsubasa is one of them. She lives in a world where there are almost no stairs or ladders, even though the buildings and space between floors is getting higher and farther.

On her first day of the high school she chose (because it had stairs), Tsubasa Fukami jumps off a landing to see if she can fly, and collides with Ren Takatsuki who was also using the stairs...

(Source: Comic Natalie, translated)