There was a point up until last year that the chat log went up until something like 500 lines. During the Community Platform update (I believe), this was changed to just display the last 50 lines in the chat log. This makes it pretty difficult to keep up with the chat and whispers, especially in rooms with lots of spam, modules with chat messages sent often (#pictionary as an example), and such.

Personally I'd find it useful having the log go back to at least 200 lines.