XML code:
<Window Id="WndSubclass" Version="1">
<Attributes>
<ShowInTaskbar>false</ShowInTaskbar>
</Attributes>
<Position Width="0" Height="0"/>
<DialogTmpl/>
</Window>
That's the one out of Interface Writer, but I think the subclass windows in my other script are the same.
Something else, though: in some scripts, I'm using the subclass window to show dialogs and stuff (so that they have a matching custom icon, etc.), but that's not too important if I can't do that any more.
