TabCompleteEvent does not fire when entering "/" or the beginnings of any commands in chat, but will fire for the arguments of an existing command.

For example

"/"

"/ver"

"/hel"

will not fire the event, but

"/version "

"/help "

/help te"

Will fire the event.

All of the above cases have been confirmed to work fine in spigot 1.12.2, but not in spigot 1.13-pre7.

simple code to test with:

    @EventHandler
    public void onPlayerChatTabComplete(TabCompleteEvent event)
    {
        event.getSender().sendMessage(event.getBuffer());
    }

which should print the buffer used for tab completion, but will not print it when tab completing commands in 1.13-pre7.