How the chatbox visually looks, and how the player interacts with it has not changed much over the years. Command lines are shoehorned into the same functionality. WIth jj2+ becoming bigger and bigger and with more commands and shortcut oddities, it can get quite daunting.

Autocomplete for commands.
Commands can be difficult to remember. The current chat box is 1 line, and last chat of 8 lines above it. When starting Chat with / to start your command, moves the 8 lines of last chat up-to 5 upwards to show 5 autocomplete answers, alphabetically on available answers left. So it should look something like this:

Player1: Hey
Player2: Hey
Player1: How goes
Player2: Things go well
Player1: I'm testing the chatbox
||Player1 captured the flag
2 minutes left...
Player2: Cool
/addadminip <group> <ip_address>
/addallowedcommand <group> <command>
/addblockedcommand <group> <command>
/addprivilege <group> <privilege>
/admingroup <group> <on|off>
Command: /a

5 seconds later.....

Player2: Things go well
Player1: I'm testing the chatbox
||Player1 captured the flag
2 minutes left...
Player2: Cool
/allowready <on|off>
/allowmouseaim <on|off>
/allowsuicide <on|off>
/allowwalljump <on|off>
Command: /allow

5 seconds later.....

||Player1 captured the flag
2 minutes left...
Player2: Cool
/allowready <on|off> - Toggles the availability of the /ready command
Command: /allowr

So preferably when one command remains, it takes the description truncated to the first sentence.

So apparently there is a autocomplete function with the TAB key, but it's functionality is only to find one possible alphabetic solution.

I think that feature does not really add much reason to use, except for if you know the commands well but type the word slightly wrong. A tab key can help spell out the command to correct the mistake.

That's not exactly where I am going with this feature request. This feature is meant to make commands more accessible, not to trial and error with tab or open the full list of commands. The tab key itself is also quite hidden, while a pop-up does it for you. If players can see the possible autocompletions and have some descriptions, it could be a huge quality of life feature.