When in the command block interface, when you press SHIFT + Right/Left Arrow key to select some text, if you go have to scroll farther than the box shows, then it scrolls that way but does not show a visual for it.
MC-148328
Command Block Selecting
Resolved
Duplicate
0
Thomas Johnson
(Unassigned)
Unconfirmed
Minecraft 1.14 Pre-Release 2
Retrieved
Duplicate of MC-140646.