You can always enable logging, at least for the duration of the session.
Checking *which* lines in the buffer is a completely different issue. I was under the assumption that you simply need to know if the window changed, not what changed.
If you want to know which lines are new I highly suggest handling proper events like ON TEXT,ACTION,...-- this will be much more efficient than polling every window and looping over $line(#,%i) until you find the different line.
