When entering text into chat or a command block, you can scroll through it using the arrow keys. When you reach the side of the visible text, it will automatically scroll to display more text, if there is any.

However, when you are holding shift to select text when using the arrow keys, it does not scroll and you are now selecting/typing somewhere in a line of text that you cannot see. This is very annoying when you want to copy an amount of text wider than the text box in a long command.

It is a bug as this worked completely fine in 1.13 and prior releases.

This bug was introduced when the ability to move through text and select text was added to books and signs, and was overlooked.