so its acctelly a like cool bug it got fixed on newer virsons but i just want to mansion it out am not sure what versions it works but i tried it on 1.8.9 and it worked so lets start first go to minecraft relams then press esc on your keybord then join a server fast then when you join the server its gonna show up a error then your mouse will dispere on your screen just go out from full screen and join back in full screen and your mouse will be there press ok in the error then go to multiplayer then your gonna see the backround of the server you jonied press esc and your back in the server if you dont understand the thing just do what i said