Although its great that the text boxes are now retexturable, the way they are resized causes issues with resource packs causing them to have very limited freedom in how they are allowed to retexture.

It seems that when the text box needs to be resized, it takes the last portion of the texture it needs, and then applies it to the end of the very last pixel of the texture. This means that if a resource pack's retexturing of the text box goes at all outside of the vanilla texture's border, it causes parts of the texture to be cut off.

I wasn't sure how to articulate this so sorry if its a bit of a confusing read, but hopefully the images help.

(I have since learnt that this is very customisable and I just didn't look into this properly before submitting a report. Sorry )