When an advancement tab's background image is not square, the way it repeats is inversely related to its aspect ratio. I expect the way it repeats to be directly related to its aspect ratio.
For example, the chain command block side texture is 16x64, or an aspect ratio of 0.25. With "GUI Scale: Normal", the texture repeats every 32px on the x-axis, and 4px on the y-axis. I would expect the texture to repeat on the y-axis every 128px at this scale.
On a related note, the texture isn't animated, but as far as I'm concerned, that can wait for another day.
- Download the attached advancement background_test.json and place it in the following directory of your world folder
- Load the world
- Open the advancement GUI and look at the advancement