Yep, it seems the resource pack I'm using to change a few sounds has every texture in it for some reason, including an outdated checkbox texture. Disabling it fixed the issue.
Nice, in the latest snapshot 17w50a it has been fixed. I don't know if that is going to stay but I hope it does.
Found that it will disappear at a height >130
It made text on the multiplayer screen more green and yellowOriginal:
Ghomaghity
Yep, it seems the resource pack I'm using to change a few sounds has every texture in it for some reason, including an outdated checkbox texture. Disabling it fixed the issue.