The bug

When you go down in a command drop-down list using the directional "down" arrow in a command block, you do not see what you select.
The highlighted word (to show the player what he selects) is below the list items that the player sees (see video).
This also has the effect of not being able to see the very last option in the drop-down list at all.

How to reproduce

1. Try writing a command in a command block using auto completion
2. Try to select an item further down from the drop-down list
   → You will not see what you want to select

Example

Here is an example in video: When you go down you do not see what you select.mp4