When you have a command in a command block that is so long that you can scroll the text field and then you auto complete something at the beginning of the command, the text field scrolls block to the right so that you do not see what you auto completed.
How to reproduce:
Write a long command in a command block so that you can scroll. You can for example copy this command in a command block: execute as @p in minecraft:overworld run effect give @a minecraft:jump_boost
Scroll to the beginning where you can auto complete
Auto complete it with anything
What I observed: The text field was scrolled to the right that you can not see what you auto completed.
What I expected: The text field doesn't scroll at all.
This also happens with commands in the chat, but the command of course needs to be longer.
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This has been fixed with the fix of MC-140646 in 23w31a.
Confirmed. Relates to MC-140646