You can click through the "Loading World" overlay, which allows you to open other worlds while one is already loading.

Upon selecting a Singleplayer world (And rarely the multiplayer menu too), Minecraft tends to switch worlds at random, it selects the actual world, but most of the time, it switches to another world on it's own. I also want to note that sometimes when loading a world, regardless of actual world selected, your skin doesn't load correctly, you start slowly falling through the ground, and sometimes, the world is invisible, it also takes what seems like forever to load. As if it didn't load fast enough, and got into the world instead of loading all the world assets from the dirt loading screen. I want to note that when it changes world, it can say "Loading world." then quickly disappear and reappear, that's how you know it switches worlds. Pls fix this! before 1.9 comes out! :'(