Download trackbar.pl — trackbar-soliton.pl (source) — trackbarworkaround.pl (source)

Once you’re in the flow of IRC, another important tool to keep track of
which messages are new is what mIRC calls the Line Marker. Whenever you
switch windows, it will add a horizontal line marking the position where
you switched away from this window.

That way, you’ll easily be able to refind the spot where you left off
reading the chat window when you return to it.

The mIRC original will only update the marker if you have actually
“visited” it, i.e. scrolled the screen so much that you could actually
see the marker (in case it had went off-screen). Unfortunately, there is
no such feature in this trackbar.pl version.

I am pretty sure that I once had a trackbar version which would allow you to directly jump in scrollback to the position at
which the trackbar is located. I wonder where it went. (I’m also fairly sure that I was using that mentioned version in the
last days, and not this one.)

This script has now been located and is available as trackbar-soliton.pl (source)

Another unfortunate bug of this script is that it doesn’t cope well with split windows. Then again, Irssi itself doesn’t cope very well with
these, so whatever.

One dirty fix to this problem is to instruct Irssi to redraw everything on window switch. This is implemented in trackbarworkaround.pl (source)