Thanks AlexKaras for the inputs,
In my case, AUT will be invisible during that window comes.
I tried use SetFocus() or Activate() method but the new window is not considering as unexpected window.
Log will just saying the your Main window is invisible thus can't be activated.
Well... Usually, TestComplete can access properties of the invisible object that exists. So, if the main window still exists but is hidden when the additional window is opened (what was the reason for such design..?), what if you check whether or not main window is visible before any UI action (mouse, keyboard) and proceed appropriately ?
