This may be hard to test if your system is actually decent.

For whatever reason, if your computer can't quite keep up with a shader, Minecraft will decide to ignore your input instead of slowing down rendering. Now, if you have a decent graphics card, these steps won't work for you, but they work for my setup (listed in "Environment").

Steps to Reproduce:
1. Open the menu, and press "Super Secret Settings".
2. Observe that the 'fxaa' shader becomes active. For me, this does not cause any severe lag or issues. I can play the game just fine with this.
3. Click "Super Secret Settings" again.
4. Observe that the 'art' shader becomes active.
5. If your computer is anything like mine, it will continue rendering the world in the background (including animated things such as enchantment glows), and GUI buttons will still get highlighted when you hover over them. There may be an FPS drop, but not a complete lock-up.
6. Attempt to click any buttons, or Escape to exit the GUI.
7. Observe that, regardless of how many times you click, what you click, or how fast you click, your input is completely ignored. The only input the game still acknowledges is the location of your mouse cursor, which is rather useless on its own.