If you have a computer with 2 monitor (or probably more) and you open a menu (inventory, options etc.) the cursor can leave the screen in which minecraft is open without pressing alt +tab even when in full-screen.
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Resolving as WAI since I don't see why this actually causes issues (I confirmed with someone else that mouse movement is normal on the other monitor). If there's a reason why it shouldn't be possible to do this, comment and I'll reopen it.
I think that it is not intended behavior since it is not present in older non-snapshot versions of the game (as you also said) . It is frustrating since if u click out side of minecraft in the second monitor, minecraft actually minimizes. This can certainly happen especially when you try to do thing fast in the menu that u have open
I'm pretty sure this is intended behavior, as it matches the behavior in windowed mode – the cursor isn't locked into the window when in a GUI since it doesn't need to be. That said, I haven't actually tried 2-monitor fullscreen; does the second monitor show stuff or is it blank?
Hm, actually, looks like it not being able to exit was reported as a bug in 1.12.2 (MC-122740), and then it presumably was fixed during the update to LWJGL 3. I'm still not sure which this should be considered intended behavior or not, but being able to exit seems intuitive to me.