The issue is when you press Alt+Tab on fullscreen, then return to Minecraft, the game is frozen. You can still interact with the game, but the screen never updates unless you force quit Minecraft.
MC-126395
Alt + Tab freezes the game
Resolved
Duplicate
2
Can confirm in 18w10a. For me I can move my but the in-game cursor just stays in place. Can also hear some selection sounds.