Long strings of text will not visually scroll when the player highlights the text and navigates it using the arrow keys. This applies to chat and command blocks.

What I expected to happen was...:

When given a long string of text to be highlighted, the text should scroll to keep the text cursor and text surrounding it in view.

What actually happened was...:
The text cursor goes off the screen and the text will never move.

Steps to Reproduce:
1. Put a long string of text in chat or a command block. As an example, paste in this string of text.

abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz

2. Hold both the SHIFT and LEFT ARROW keys. You will see the text cursor move through the text before completely going off screen. The text does not scroll to keep the cursor or the rest of the highlighted text in view. This also applies in the other direction.
Additionally, the shortcut CTRL + SHIFT + LEFT/RIGHT ARROW (which jumps to the nearest space) also does not scroll the text.