Inside of the Command Block interface it is no longer possible to select or copt the text of the Previous Output. Whereas in 1.13 you could click a part of the text and move around with the arrow keys, select parts by holding shift, or just select everything with ctrl+a, in 1.14 this is no longer possible. You can click it and the text-cursor will appear, but it is impossible to move around with the arrow keys or use ctrl+a to select all, let alone any, of it.

Clip showing what I mean

This is really annoying when trying to do stuff with nbt, because instead of being able to just copy some nbt I now have to manually type everything over.