No lid, no wall, no glass. Just gravity. Watch how a thick atmosphere fades smoothly into the vacuum of space.
Nothing pushes the atoms away and nothing holds them in except gravity. Each atom flies in a curve, gets pulled back down and bounces off the ground, over and over. Few atoms have enough speed to reach great height, so the density drops off smoothly, halving every few kilometres on Earth. High up there are so few atoms that we call it a vacuum, but there is no edge: the atmosphere simply thins out.
Try it: raise the temperature, lower gravity or switch to hydrogen and the atoms climb higher and some reach escape speed and leave for good. That is how small, hot worlds lose their light gases. Real air atoms collide with each other constantly, about every 0.07 µm at sea level. Switch on collisions to see atoms knock into each other near the ground and almost never meet high up. Atoms are drawn far bigger than real ones so you can see them touch, and collisions don't change the shape of the profile. Time is sped up. Use View height to zoom out and see where satellites orbit, far above almost all of the air. The preset buttons also switch the landmark lines to that world.