Yes. Custom Task Pane is created per Window. That is to say, if you create a
CTP in AddInStartup to the default document window, the second open document
will not show that CTP.
If we want to show a custom task pane for each window. We need to hanlde
WindowActivate event and keep track of all open windows and their CTP status
in a global collection. There are MSDN documentations for this objective,