leafChat has several usability features designed to help you cope with busy channels. Anything that appears in a channel since you were last in that window is indicated with a red dotted line. When you click on the window, the line gradually fades away.

If there's so much text since the last time you viewed a window that you need to scroll up to read it, no problem - the red line won't fade unless you are scrolled right down, so you can just scroll up to find it and read the extra text. And, so you don't forget, the window has a red shadow at the bottom whenever you are scrolled up.