Modded servers might be able to use tab-completion suggestions to trick players into sending incriminating messages

The bug

Malicious modded servers might be able to use custom tab-completion suggestions together with a mandatory resource pack to trick players (using vanilla clients) into sending messages that contain objectionable content. The resource pack could use negative font spacing to hide the objectionable content (similar to the resource pack provided in MC-254485).

How this would work

1. The player types the start of a whisper command into the chat-bar:

   /w 

2. Instead of suggesting just the name of another player, the server suggests the name of another player with harmful content appended to the end:

   /w pine1needle (You're a INSERT_SLUR_HERE, but I guess I'll talk to you anyway.) 

3. The harmful content inside the parentheses is hidden by the mandatory resource pack, so to the player, it looks like a normal command.
4. The player uses tab to insert the suggestion into the chat bar.
5. The player types a message and sends it. The message they actually sent contains harmful content, even though they didn't see that content.