Excel Modeless UserForm - visible only when the activeworkbook when launched has focus

Logic for creation of a Modeless UserForm exists in a central XLAM that my users have access to.  They launch it from a button on a custom toolbar created by the XLAM.

Modeless is necessary since users will have this userform open to assist them with various interactions of their workbooks.  However, if the user switches to a different Excel workbook I would prefer that the userform no longer be visible (since it is only intended to be interacting with the specific workbook that was active at the time they launched the user form).

      1 - So userform logic lives in File (A) - an XLAM
      2 - User launches a separate XLSM file - File (B); and while (B) is active, they launch the user form from (A).
      3 - User uses Userform to assist with their interactions with File (B).
      4 - If user decides to switch focus to a different workbook - File (C), they should no longer be able to see or use the UserForm; since (B) is no longer the active workbook.

I'm ok with either hiding the user form while the currently active book (File C) does not equal the original book that was active at the time launched (File B); or with unloading the userform as soon as they leave the book that was active at the time launched (forcing them to relaunch it when they later come back to the original book and want to again use the functionality on the user form).

Does anyone have suggestions?  So far most of the examples I've been able to find have dealt with tieing the behavior into ThisWorkbook Deactivate/Activate but i don't see how that would work since the userform logic lives in a standalone book (File A) separate from the original workbook that was active (File B) when they launched the userform.  So I don't think This Workbook would work since the userForm code doesn't actually live in the file whose active status needs to be monitored.

Any guidance would be greatly appreciated!
Thanks