In 1.19.3 and lower, buttons with a different width than the texture rendered with the seam being in the middle of the texture.

In 23w06a, this was changed, which also caused MC-259831.

In 1.19.4 Pre-release 2, this was changed again, which fixed the issue the reporter of MC-259831 was having. After this change, 20 pixels of border on the left and right sides of the button are preserved, while the middle of the texture gets repeated.

In 23w31a, this was changed again: it is now a nine-sliced sprite, however it only has a border size of 3. This reintroduces the same problem as MC-259831.

I believe it would be more friendly to resource pack artists if the default \assets\minecraft\textures\gui\sprites\widget\button.png.mcmeta file was changed to make the effective UVs of buttons rendered by the game better resemble how it was in 1.19.3 and earlier. This could be done by changing its left and right borders to a high value such as 99, although it would look ugly for buttons longer than 200 pixels if the texture is not changed to be wider.

Fixing this issue should also resolve MC-264573, although I do not believe these are duplicate issues as that one is comparing just the center of the button UV in 23w31a to how it is in 1.19.4 to 1.20.1 rather than in 1.19.3 and lower.

Attached are screenshots using a custom resource pack made to demonstrate the button UV in the different versions, as well as the source texture files used. The button.png.mcmeta used for the 23w31a test was copied directly from the default files.