When loading or generating a world, the panorama background will continue to move normally for a few seconds. However, when the progress bar appears and the world actually starts loading all together, the panorama will stop for the rest of the loading / generation sequence. This is not the right behavior.

The intended behavior should be that the panorama background keeps moving while the world loads / generates.

Steps to reproduce:

1 - Generate a new world or load an existing one.

2 - Watch the menu's background closely. Notice that it stops moving when the progress bar appears.

This bug is very old, and has been in the game since version 1.2.10.