When you collapse all windows of a session (alt+leftclick, alt+f1), $window().icon returns "off" for hidden (collapsed) windows correctly, but $window().state doesn't return "hidden". Either way, there isn't a distinct method (at least that I know of) to know if a session is in collapsed state, or if any window is part of a group or not.
$window().state should return "collapsed", or maybe a .collapsed property should be added.

And now that I'm here suggesting, I'd like a command to collapse/expand sessions too. So I could avoid new query windows not being part of their session group, for example.

I would leave the .icon giving its original info, as the window still has an icon, even if u cant see it right now. But i agree with the rest, that there should be methods to check the state etc.

About .icon, well not really. This property returns on/off depending on whether the icon (button) is visible, according to mirc.hlp. When buttons are collapsed, only the status button is visible. So, this is working correctly, at least for me and the help file.

well thats the second stuff up i made today, i didnt even check what .icon did, i assumed it identified the icon in use. My error.