In any Minecraft world at X: -1, and any Z value, whilst F3+G is active there is a line of blocks that are not in a chunk. It's just in a line of blocks with no connection to any chunk that goes on to the end of the world.

In the photos below the chunk on the right has a chunk X value of -1, while the chunk in the middle also has a chunk X value of -1. Meaning that the chunk on the right must be connected to that line in the middle yet still not indicating that it's part of it in F3+G. This indicates that the chunk to the right is a 17x16 chunk instead of the normal 16x16 seen across the rest of the world.

================== Steps to reproduce ================

1. Go to X: -1 Y: ~ Z: ~
2. Press F3+G

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I am not a hundred percent sure this isn't some feature to indicate x: 0, but it seems pretty fundamental to have the entire MC world all separated by chunks that are 16x16 blocks each. Which makes it really weird if this was a feature to have this line blocks that make the chunk next to it 17x16. I'll let guys look at this from here.