Yeah it isn't like that for me, it would just go on normal resolution when I press F11, but yeah it's most likely the resolution u use and if your screen/graphics card supports it, thank you for the help though! This actually is useful for me since now I know to install mods and forge in an easier way with the help of MultiMC.
Thing is, I mostly use 1152x720 since it isn't that stretched like 1152x864 is and looks nice in-game, but I don't like having half of my tool/weapon/item cut off on the screen, I like seeing the whole thing, I might just go windowed or stick to the resolution depending if I'm playing on 1.8.9 or playing survival on 1.15.2, but thanks for the suggestion though!
Thank you for the reply and help, but just a question, will it launch minecraft in fullscreen mode with the resolution? I'll still try it out, sorry for the late reply haha. Edit: What about optifine, I kinda wanna use it for the fps boost since mc isn't playable without it for me. Edit 2: I solved the optifine problem, it doesn't work on fullscreen though..