Even when the game is in fullscreen, my mouse gets out of the screen and goes to my other monitor or opens up the taskbar when I'm looking around in a world. I've seen lots of issues like this in other reports, that are open since this wasn't even on the 1.8 version, but even so, this isn't fixed in game yet. 

I don't think solving this is difficult: it would be, at maximum, 10 lines of code saying that when in a world, or when controlling the avatar, the mouse should be locked in the center of the screen, while still detectingthe movement we make on the mouse.

Searching solutions to this, I found out that many of the reports of this case were ended, saying it was a duplicate of another. And in none of these, duplicates or no, was there a solution found out by the developer. I tried some other ways to solve that using external programs but none of them worked out perfectly.