So this isn't just a Minecraft problem, this happens to a lot of other softwares I use on my computer as well. The problem is, when I press a key in Minecraft, even if it is in Fulscreen Mode, it will affect outside Minecraft, and its really annoying when you wanna focus on the game. So is there any program or anything that would kinda isolate Minecraft. Often I can't press some keys because it would send the keystroke to some other program instead of Minecraft. I use Linux Mint (19 I think). Thanks for spending your time to help me.

Mint ships with multiple DEs, do you know which desktop environment/graphical interface you're using? This sort of behavior is likely a DE/WM issue, not a "Linux problem".

I use Cinnamon, which one would you recommend? What if I try and assemble one myself?

Are you saying the key is defined in minecraft, and does what it should, but also does something else outside minecraft at the same time? Which key? What does it do?

And what other software reacts like that?

Yes, I think any key, but the key that annoys me the most is the SHIFT key. Apparently in Linux Mint, when I press SHIFT + [any alphabet key] it opens a tiny search bar kinda menu, I haven't found out what it does yet, coz when i type something in and tap ENTER, it does nothing. But in Minecraft, SHIFT is for crouching and Q for throwing items. So if I was to hold shift, I will crouch, but then if I type anything else while holding SHIFT, it won't register in Minecraft. Now I wouldn't mind much if in single player, coz then its a very relaxed gameplay. But I play a lot of competitive minigames on Minecraft servers, and being able to quickly react and respond is key to winning the game, an this thing annoys the heck out of me.

Here's a screenshot of this menu (this menu is the main troublemaker, but there are things like the start menu which gets toggled even if I'm in fullscreen mode, so generally if I could stop Linux from giving the key presses to anybody other than Minecraft would be great.

2 approaches:
- search minecraft documentation if you can't configure it to grab all keyboard input and not pass it on. Might also be a java option, not limited to minecraft
- check all hotkeys and keyboard shortcuts in LinuxMint, see which conflict with your minecraft usage, and change or disable them.

FWIW, I'd never use Shift+... shortcuts globally...

aah, thx. I can't find anything about this in their documentation, and there are too many global hotkeys using shift, also these are only the ones I manually found, since there isn't a feature to filter the list of hotkeys. I would rather have a way to stop Minecraft from passing on the keystrokes. I'm gonna keep searching. Thanks for replying.

Yes this is not only the problem of the games.It occurs on other software also.I am also facing this problem on many software