When you press tab, it is supposed to list thru all options, clearing the old list for the new one. Instead, it leaves a large amount of text behind if you have more then 1 line used.
To reproduce this:
Type the / character. Then type tab. Note that there are a lot of commands.
Press tab again. The list is shown again.
To see this better, try /scoreboard teams option asdf color and press tab.
It works fine.
Press tab again.
Note the mess it makes.

This is caused (probably) by the method

this.mc.ingameGUI.getChatGUI().printChatMessageWithOptionalDeletion(stringbuilder.toString(), 1);

on line 275 in GUIChat.java (mcp names). It is creating a line of chat that will be overridden, but is only applying one line even though the message may be multiple lines long.