When first opening the Chat window, some users may see old chat lines displayed. Depending on your usage... this may be a desireable thing, or not.
You can make the chat box appear blank when first opened... by setting the Max History setting to zero (0). Doing this will erase all old chat lines for each new instance of the chat.
Alternately, you can set the Max History to any number you wish... and that number of current/old chat lines will appear in the chat box when it's first opened.