The bug

Since release 1.14, I cannot select nor copy command block output to the clipboard. I used to be able to use the cursor keys on the keyboard along with the 'shift' key to select command block output, then press <ctrl + c> to copy the highlighted text to the clipboard, and finally press <ctrl + v> in a text editor to see the whole output result.

How to reproduce

1. Create a fenced in area, and spawn a villager in it
2. Put a command block outside of the fenced-in area and give it this command:

   /data get entity @e[type=minecraft:villager,limit=1,distance=..8]
   

3. Put a lever next to the command block, and pull the lever
4. Right-click the command block and look at the command block output, and try to select the text within the part that says Previous Output 
   → You can do it successfully if it's 1.13.2, but you can't do it if it's 1.14.x