How to detect when the user moves a stack

[jbv at souslelogo.com]

Hi list,
How can I detect when the user moves a stack around the screen by 
clicking on the top bar while leaving the mouse down ?
Here's my problem : I have a main stack calling a modal stack that 
displays information in real time while some client / server dialog is 
running. When the modal stack opens, it is located at the loc of the 
main stack. But if the user moves the main stack, the modal stack stays 
at the same place, and I would like it to move accordingly and remain at 
the loc of the main stack, no matter where the user moves the main 
stack.
At least this is happening in the IDE, I haven't built a standalone yet.

Thanks in advance.
jbv