Image around the skybox doesn't exactly match together

I’m currently making a panorama for my website’s background like the panorama behind Minecraft’s loading screen, and to do that, I follow an old tutorial to get 6 of the required image for the panorama to work:
[Texture Creating]Creating a panorama for the main menu in 1.8 - Tutorials - Archive - Minecraft Forum - Minecraft Forum
and also another tutorial on YouTube to create the skybox. And it did work, but when I move the camera, the floor is bend just small enough for me and most people to notice.

Old source code:
Source code after fix:
Is there a way to prevent this bending?
(Sorry for my bad English spelling)

One option is to use a box of segmentation greater than 1. For example: let skyboxGeo = new THREE.BoxGeometry( 10000, 10000, 10000, 10, 10, 10);
Another option is to use a cube texture for scene.background: three.js examples