The bug

In a command block, when you go down in a command dropdown list using the directional "down" arrow, 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.
This also has the effect of not being able to see the very last option in the drop-down list at all.
This happens both when going up from the first option and when going down.

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 dropdown list with the arrow keys
   → You will not see what you want to select

Example

Here is an example in video: MC-140785.mp4