I don't know if the summary was clear enough...

Anyway, changing resource packs can cause text longer than one line to be displayed on one line. This happens if the text did fit on one line before the resource pack change. This also happens when toggling the "Force Unicode Font" option. Here's an example:

Steps to reproduce:

1. Turn on Force Unicode Font.
2. Type some text into the chat so that it almost wraps to the next line.
3. Turn off Force Unicode Font.
4. Look in chat. Your previous message is too long to fit on a line, but does not wrap and continues right outside the chat window.