Tab-completion in command blocks no longer works

The bug

It no longer is possible to tab-complete commands (or their arguments) in a command block. This happens both when the command is and isn't prefixed with /.

How to reproduce

• Place a command block
• Type /play and then press tab ⇒ nothing happens
• Replace the text with play and then press tab ⇒ nothing happens
• Replace the text with /playsound minecraft: and then press tab ⇒ nothing happens
• Replace the text with playsound minecraft: and then press tab ⇒ nothing happens