The recent snapshots have been displaying an irritating glitch with the chat bar. If you open chat with the [T] key you'll get a "t" as the first character in chat, which you have to backspace. If you open chat with the [/] key you'll get "//" in chat and you'll have to backspace once before you can start typing a command.

When I change my launcher to play in 1.12.2 this problem goes away completely. When I play on 18w08b (and the last few snapshots before it) this happens every time without fail when opening chat.