I didn't know about this one, thank you for posting.
Also, on most windows, you can option-click the green window bar button and get this same behavior. However, some apps, like Safari, will instead expand the window to "a reasonably large size" that will not necessarily cover the whole screen.
I have zero tolerance for wasting time watching long YouTube videos and I didn't think I had any interest in historical ship construction, and yet I couldn't stop watching all 40 minutes of this. Amazing.
If you're noticing the same thing I did, he's doing that because he's working on OpenGL shader code that's embedded in a string inside his Dart code. In this case, the IDE only sees a plain vanilla string, so Notch has to indent the GLSL manually.