How do I create a live tiling effect like this?

[Marscaleb]

In this video you can see a guy creating a tillable texture, and as he draws it, it automatically displays it as being tiled.

https://youtu.be/mMZ3Mn-esOI?si=m2v7_BHJJDF0y-6v&t=152

How does he do that?  It's clearly in Gimp, and this would be SUCH a useful tool.

[denzjos]

Set the play speed on the video to 0.25 and you will see that he used the gimp symmetry dialog.  

https://docs.gimp.org/en/gimp-symmetry-dialog.html

[sallyanne]

Some of that looks like Gimp's mosaic effect too