Command block autocompletion doesn't work without leading slash

The bug

Pressing tab to autocomplete does nothing in a command block unless the command is prefixed with a slash.

How to reproduce

1. /give @p minecraft:tile.command_block and place it
2. Open it, click the input, and type "sco"
3. Press tab
→ Nothing happens
4. Delete what you typed and type "/sco"
5. Press tab
→ "/scoreboard" is filled in